What is a medieval technology method of underwater enemy detection?

Question

I am writing a DnD module where fishmen are invading island nations. The islands would naturally have adapted to this in some way as the raids have been happening for years. What method would these island nations use to detect underwater invaders?

I first thought of nets with floats and bells but these would give constant false positives from fish or tides.

Magic is an option, but needs to be a realistic and low level spell that a run of the mill spellcaster could accomplish (say 3rd level or below). I considered a water-breathing spell cast on a regular patrol but the underwater terrain is unlikely to be hospitable to a normal patrol and visibility would be too limited to be useful.

I much prefer a more generic non-DnD specific answer but would settle for something in the DnD world.

Answer 1

Marine watchdogs.

First option:

Build a net wall around the city in the water like torpedo nets. Inside the net give a cautious and jumpy sea dweller a home. The net has no warning function, it simply isolates the sea dwellers from predators so that they are moving freely around the net.

If something is inside their waters, the sea dwellers are fleeing into provided homes which can be conveniently watched. So an intrusion will be detected faithfully.

Second option:

Train marine animals to warn people or even track and attack intruders. In our world dolphins or sea lions are trained e.g. by the US Navy.

Answer 2

Don't live directly next to the water.

You can still be close enough to the water to easily hop in your boat and go fishing. Just build your house a couple hundred meters inland from the high water mark. This is just a good idea anyway. This makes it harder for your house to be washed away during a storm surge. Then any fishpeople will have to come out of the water to threaten your village. They can then be spotted by guards or lookouts as normal.

If the attacks by raiding fishpeople are a regular occurrence you build a wall around your town and you treat them like any other raiders that come from the water.

Answer 3

Your idea of nets with bells was a decent idea, but as you say, fish and tides and such would ring the bells. Plus, an intelligent creature would cut the lines before an invasion.

I think you might want to consider a more active defense.

Caltrops for example would be useful in the shallows, when your attackers must transition from swimming to walking. A few spikes in the seaweeds will slow down the attackers.

If those caltrops are also sitting on bladders of air, then the pressure would pop the bags. The rising bubbles would be visible to anyone watching the water's surface.

Another approach is tarred spears. Fire-harden a wooden spear, then coat it in tar to waterproof it. If the spear is 10 feet long, bury it at a 45 degree angle pointing out to sea so that only 3 to 5 feet are above the seabed. Build a wall of these spears, leaving only your fishing boats' exit path clear. This wall will slow them down, but not stop them...

Strings or wires can be tied to the shafts below the sea floor (so the wires are not exposed) and the other ends tied to bells above the high tide line. When the creatures either impale themselves on the spears or rip the spears out of their way, the bells will ring.

Build a sea wall so that once they come ashore, they then have to climb a wall. This wall should be strong enough to handle storm surges and high winds. It should have gates, of course, but those become choke points at which you can slow their invasion. Of course, the wall also protects your village from actual storms, so double win there.

Defense in depth. Use all of the above (and any other ideas you come up with), not just any one idea.

Answer 4

Here is something that might be slightly strange, but could work for you.

Guard posts built on Glass bottom buoys.

You will have to set, as a precondition, very very clear waters as a starting point. Set a guard on each buoy so that they can use the glass floor to look out into the rest of the lagoon. This will help cope with wave chop distorting your line of sight.

For night duty, design a floating, glass bottomed lantern, maybe with a mirror to focus a beam out into the water.

Make sure the observation posts are out beyond where the waves begin to break. Regular boat patrols to help. In case of storms, bring everyone in from the outposts and put them in bunkers on dry land. It's not as good for observation, but you won't want humans on a little buoy during a squall.

All you are really doing is a standard guard patrol picket. You are just making it a bit easier to see the fishmen coming by smoothing the "surface" you are looking through to avoid problems with refraction, and adding some primitive underwater lights.

Medieval glass wasn't always of the highest quality, so you might have to use a little bit of magic on the glass during construction. After that, no contiuous scrying or spellwork required.

Answer 5

The Alarm spell is a 1st level spell that both Wizards and Rangers can cast. It's intended purpose is to notify you someone is where they shouldn't be.

Unlike mundane means, you can freely specify things like, "Fish don't trigger it."

As it's only a 1st level spell in theory even a scrub would be able to learn how to cast it with a bit of learning/practice. Rangers don't have schools and are usually self-taught. It would not be impossible for the local wise-man or grizzled hunter to have this trick up their sleeves and the position probably would even be a traditional role in the village if this sort of thing happened regularly.

Feats like Magic Initiate and Ritual Caster in particular make it plausible for even someone not well-versed in the arcane to know the spell if they study enough. You need not be a learned Wizard or a world-wise Ranger if it's just something that gets taught along a family line for example.

I realize you wanted tech but I didn't see the signature "Ward this area" spell listed in the answers so I thought I would mention it.

Edit: Upon further review, the Alarm spell itself will not help you. However, you can make spells of your own and if you have an entire nation's worth of resources and a reason to create such a thing, making a larger-scale version is not impossible or even improbable in the slightest. It does require you to dive out of the rulebook but such a thing is not impossible or even frowned upon. Part of the core premise of Wizards is unearthing and creating new spells to meet needs.

Answer 6

Early warning underwater acoustic mirrors.

http://www.andrewgrantham.co.uk/soundmirrors/

These amazing and amazingly huge parabolic acoustic mirrors were built in England (Germany too, I think) because they could amplify the sound of an airplane in the distance. Radar made them obsolete, but they are still there. Check out the linked page.

The bigger they are, the lower the frequency they can detect. From https://en.wikipedia.org/wiki/Parabolic_microphone

This is because, from the Rayleigh criterion, parabolic dishes can only focus waves with a wavelength much smaller than the diameter of their aperture. The wavelength of sound waves at the low end of human hearing (20 Hz) is about 17 metres (56 feet); focusing them would require a dish much larger than this.

I envision these things underwater. It is D&D so maybe they could be made from giant shells. The people in the 1930s who built the dishes shown had electric microphones, but one can transmit a sound signal along a taut string, in the manner of a tin can phone.

Listeners topside would listen via a taut string running from a pickup at the focal point of the parabolic acoustic mirror back (possibly through a hole in the mirror) up to the listener. I envision the listener with his head up inside a fixed bell-like helmet, with multiple strings running to it.

In case you wondered: the listeners are listening for fishmen sounds. They might hear a lot of other interesting underwater things too.

Once they heard fishmen sounds, in addition to sounding an alarm they could mute one incoming wire at a time and by a process of elimination determine which direction the sound was coming from.

Answer 7

How real cities handled their ports

Ancient cities tended not to be located on the water, but rather a few miles inland with a client village on the water. This way, they had the utility of ocean access, while still being protected. If the village was lost, no big deal.

As cities got larger they tended to connect the sea-side village with the city via walls. This new and larger city was surrounded by walls on all sides, but often had an inner wall between the port and the actual city. This allowed for greater protection against seaside attacks. Not to mention the port itself may be walled off into sections. These sections might mean that there are 2 harbor areas or the open area only holds ships. All houses and warehouses being behind walls in the 2nd harbor area.

Further improvements were the usage of a sea wall, a wall that literally is standing in the water, so besides the entry to the port it is completely protected. Even the entry typically had a chain possibly with logs that was strung across it. These methods protected the main city, and while the port was not totally safe it was reasonably safe, at least against ships.

Your World

In your world, biology and the usage of magic skews things a little, so while the above would still be used you potentially need to add a few additional features. The levels of precaution being reliant on how much money and effort you are willing to invest, and whether the burning of a part of the port concerns you that much. Also, whether these merfolk carry weapons and armor or are simply aquatic naked people.

Detection

Mundane

At night a large chain net is spread across the opening of the port. It might be possible for someone to squeeze through but they would not be able to carry much if any equipment.

With glass: Have a few buoys with bells that are towed right inside and outside the entrance to the harbor. Divers with diving helmets or a diving sphere sit at the bottom and ring the bell if they see attackers.

Without glass: Build a few thin towers, basically thick poles, with a wooden tube carrying air to the bottom of the harbor. At the top of these towers are large bells. Divers suck on the tube and watch the entrance. If they see attackers they pull on the rope and ring the bell.

Buoys are more flexible, but ring faintly. Being rung will be louder than waves though. Towers cost more, are inflexible, but will not ring from the tide.

Magical

Same as above, but with water breathing. Maybe have "detect life" active.

Neutralization

Whether your solution is mundane or magical, you WILL need more towers and many more guards. Most ports would have towers, but most would focus outwards. You need towers facing inwards. They only needed a few guards as ships were seen from far away and the chain at night kept them out. Now you need many more guards, as people are harder to spot and keep out than ships.

You need towers to face inwards and have archers or mages constantly watching the harbor. If merpeople attack they need to immediately start firing. Depending on how heavily armed the merfolk are, you may need to sacrifice the particular harbor segment.

Throw pots of pitch, oil, or greek fire onto the water and light the surface. The water will catch fire either driving the merfolk onto a narrow dock or piece of land. The small area and many towers will create a killing field. Alternatively, they will have to retreat as the water will heat and cause burns and possible oxygen problems.

Conclusion

Unless the city messes up, is heavily outnumbered, the merfolk are somehow heavily armed (unlikely), or the merfolk can throw around a ton of magic or explosives, the city will be safe. The port and ships may be lost or raided, but the merfolk will pay dearly for it (seriously they will die).

However the cost of employing so many guards will mean that only cities can do this, as villages simply do not have enough people. So maybe back to the split coastal and interior villages.

Much more effective and safer for merfolk to raid small villages who cannot afford to designate standing guards, regardless of detection. Merfolk could also swim up river to unsuspecting villages.

Answer 8

The same way they detect schools of fish, a person on a tall cliff or tower looking down, the more vertical their line of sight the better and farther they will be able to see into the water.

From above you can see well, the ocean around islands is quite clear. Your big danger times would be at night. Here is an image looking down on a bay from a cliff, note how you can see everything on the ocean floor.

Answer 9

In the realm of simple magic: Great White Shark patrols.

The magic is to give the sharks a predisposition for patrolling the shore; they can go off to hunt for food, but the magic means they just never get tired of hunting the shore near the city, even if they never catch anything there.

Similar low level magic to attract them into cages and calm them, where they can be enchanted, perhaps even attaching an amulet to them or something like that. It should not be much more difficult than an infatuation spell. If you attach the amulet, than simple touch magic to a sister amulet can call the sharks in an emergency, or send them off to hunt elsewhere during the day so your own fishermen can get their jobs done.

Answer 10

The society could employ trained Seals for monitoring the shore lines and deeper oceans for imminent attacks. We know that seals can be trained to perform stunts, ball games. I am sure they can also be trained to swim back to shore to inform a trainer of fish-people presence. After all seals are dogs of the sea.

Training of some sort of seafaring animal to alert of danger. It does not have to be seals. It could also be dolphins as they are also intelligent animals.

On Land we currently use dogs for similar purposes in our gardens and the such like.

As seals are important/as important as dogs in this society they would also selectively breed them to obtain traits they desire i.e. size/power/intelligence/speed.

Answer 11

You can defend your islands utilizing Brine pools or Carbon Dioxide lakes. These two naturally occurring underwater geographic formations that are heavily populated by extremophiles.

Brine pools are known for killing fish and crabs that go into them due to their salinity(3x to 8x higher than the ocean). These Brine pools are higher density than the surrounding ocean and swimmers would most likely be pushed out if on a normal swim. However if it were an armed raider walking along the floor the metal he is carrying would cause him to sink in.

Carbon Dioxide lakes happen at much deeper locations in the ocean than Brine pools. The depths typically needed are 10,000 ft below sea level as the temperature is low enough that liquid carbon dioxide is denser than salt water and can be collected in pools. There are other ways to make them, like volcanic underwater geysers that release CO2 into underwater holes. Fish take in oxygen rich water in order to breath, and I would imagine spells that allow breathing under water do the same thing, using CO2 lakes the raiders would suffocate underwater, just like they not able to breath with a spell.

You would need to hand-wave some of this in order for them to form close to an island but both would work well for defending against underwater attacks. As added bonuses, neither should interrupt ship travel, so normal traders should never even notice it there. Also, due to lack of oxygen Iron weapons dropped by the raiders into either should stay in relatively good condition for collection later.

Answer 12

Walls and Bells

A medieval population will be mostly composed of farmers/fishermen/... that's a lot of people scattered throughout during day time who, if not warned early enough, will be caught by the invaders.

The typical medieval strategy for protecting those people is to have a fortified place where people seek refuge in when the alarms bells are rung.

The problem, of course, is to be warned in time.

Note: a corollary is that should people sleep behind the walls, there is no need for early warning at night; thus the solution need not be concerned about illuminating the night, which would be a real hard problem.

This answer will therefore focus on a simple strategy: force the enemy to be visible early, delay it to give time to people to run to cover.

Seawall

Watching a large area, such as the shore of a whole island, is highly impractical.

If you have cliffs, that's great, otherwise I advise building a Seawall such as Pondicherry's all around the island, minus a few select passages for harbors.

This wall need not be smooth, or elaborated, it's fine to just pile on somewhat large stones. It's also fine if they are not too stable. They should however have their feet in the water, or close to, as the goal is to prevent the invader from getting an easy toehold on the island. Climbing rocks while waves keep smashing you against them is not an enjoyable experience. Bonus point if algae make them slippery.

Now, you can build watchtowers at regular intervals along the seawall, and you can also place a variety of devices on the wall to further slow down a foe: for example, transforming some holes between the rocks into inhospitable places with caltrops and other such implements. And it's fine if they rust due to the salt, just replace them once upon a time.

Harbors

This wall will have holes, for ships have to pass.

Each such channel will need watchtowers and an iron net that is raised until a boat tries to pass.

Atoll

If this does not provide an early enough warning, the next level of protection would be an atoll surrounding a lagoon. The goal is simple: force the fishmen to pop above water even if for a short period of time, so they can easily be spotted from the shore.

Note that it only really matters during daytime, as explained above, so there is no need to have torches or anything on the atoll. It's even fine if it's partially submerged as long as it's shallow enough.

If there's no natural atoll to build upon, this might be a challenge though. Sinking tree trunks, mostly upright, could build such a barrier without having to carry stones to sea. Try to aim for shallower areas.

Of course, a few channels will need to remain open if ships are intended to go at large; apply the same solution than the one for channels in the seawall: watchtowers and iron nets.

Note: those watching from there, and fishing/traveling in the great sea, would be at greatest risk; they would also be in the best position to raise warnings early.

Answer 13

surround your island of aquafarms full of fish afraid of underwater fishmen. if the fish jump out of water, that means the fishmen are near them. U could use watchtower to monitor them..

Answer 14

A reef surrounding your island would make it hard for intruders to cross unless they knew where it was, and used the proper direction to enter, to avoid splintering their boats.

The shallow water over the reef means they could stand on it and not even cover halfway to their knees. Sharp spikes and poison/bactiria make this undesirable. You would also be able to see them crossing on the surface. Maybe some of the coral is bio-luminescent, so even during a moonless cloudy night, they would be illuminated from below, and silhouettes as they approched shore.

Poison/electric Eels and stonefish could inhabit the reef.

Sandbars could randomly come and go, only the locals know them well enough for navigation. Again, crossing on the surface makes you visable.

An atol could surround your island. (A round hollow island) So no boats or even fish can enter/leave except in known guarded locations. Your villages could be on the center island. The outer island, the atol, would be covered in guard towers. Boats would have to come ashore, and be carried to the water again across solid ground, troops would have to walk. All visable in the watch-fires. (Flamable fish-oils?) This would have similar protections that a moat would have on a dry-land castle.

Also, a wall surrounding the edge of your main island, with boat-gates near the openings in the atol for convenience, OR... far around the oposite side so you have to swim, or float around the outside walls, rocks and buring oil dropping on you, to get to the next gate to come in further. Kind of like an aquatic version of Minus Tirith, for all the Lord of the Rings fans out there. Multiple rings with few gates means the enemy will spend A LONG TIME floating below your walls unable to reach up and hurt you.

Answer 15

Two ideas: fish oil, and fishing nets.

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One component of visual detection could be fish oil. Or olive oil.

While glass-bottomed boats/buoys have been suggested, the problem with glass is that it requires a lot of glass.

When I was a child in Corfu, the Greek fishermen wanting to see into the water would pour a tiny splash - no more than a teaspoon - of olive oil onto the ocean surface (Olive oil being the universal panacea for everything in Corfu, used for everything from treating skin ailments to lighting fires, treating wood, a lubricating machinery, preserving food, rustproofing, frying, you name it). This would quickly spread out, to form a molecule-thick layer atop the water, completely changing the optical behavior of the water surface nearby, so that rather than just the reflective, sparkling ripples, you could suddenly see clear to the bottom.

Having it contained by a hoop would improve the efficiency, but it holds itself together pretty well anyway, remaining for minutes at a time, at least in the relatively calm waters of the Ionian sea.

Basically it just reduces the reflectivity and fractal choppiness of the water surface. I think the layer of surface-tension on the water is very refractive, and the oil prevents this, even in the tiniest of doses.

It works surprisingly well, like a magic spell that gives you an instant window into the water to see the glittering diced-rainbow of the fish swirling below.

[To be absolutely clear: this is a tiny localized film around the spotter, who would need no more than a couple of flasks per DAY, not an environmental disaster where the sea-life will need cleaning and oil will need containing within booms and so on.]

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That said - any form of underwater detection is doomed to not be as good as overwater detection. Guard buoys or towers, and balloons in the shallows as suggested by another answer, will give you, at best, a few extra seconds, as they come in from the shallows. Compare this to the hours you'd get using overwater sight to see approaching ships.

I'd recommend three layers of netting around the entire island, or in an arc around populated villages/towns. Inner and outer layers should have diagonal mesh, and floats somewhat below the water level, but with flags projecting up above the water, such that if the mesh is cut, they bob up, but they cannot be removed downwards without being noticed.

The middle net (protected from fish by the other two layers) should have bells on, and be covered with fishhooks, sea urchin spines, etc to snag on people trying to sneak through - if jangled, then people are busting through fast, rather than cautiously.

Even this will give you perhaps 30 seconds' extra warning, a minute at best.

Answer 16

Looking at the magical ways:

Drop "special" items in the water that attract and enrage sharks. Drop them in circular paths like fences.

Teach the locals "shark friendship" and/or "undetectable to shark" (not just invisable....)

Also:

Illusion of a fake village... Running, screaming villagers. Phantasmal village guards with spears/crossbows. The villagers use it as a tv/soap opera everyone adding their own ideas.... or avoiding it's bore and drugery.

The real village is over that hill right there... The path to get there is behind that rock illusion. Oh yeah.... and an illusionary shield that looks like just another mountain.

And if someone get past? Well... invisability on buildings when seen from outside town... you have to touch the inner sidewalks before you can see them.

Answer 17

Are their sea-elves in your world? A friendship with them would be benificial.

Could they owe you a favor from a long ago war? You saved them? They hurt you, and are making reperations? Maybe just a few of them? A dozzen?

Maybe your island has something they want? A natural resource? They regularly send trading parties, with security to protect their "treasure" The Fishermen could also be after this resource, The elves HATE the fishermen and their raiding parties.

The elves protect you because they cannot refine the resource themselves, it has to be you? Maybe it's poisonous to them in its natural state, they rely on you. Maybe it's poisonous to you too, and you are being used as cheap labor? Maybe they let you manage it because the fishermen treated them badly in the past, its the right thing to do?

Maybe they are protecting SOMETHING ELSE associated with the island, some native fish or dolphin who ONLY feed at your shore or reef.

Are the fishermen sea-elves themselves? This could be a rival tribe, race, or species.