Merpeople, along with other similar underwater humanoids, are often depicted as living in great cities beneath the waves. These cities have many positive things going for them: plenty of real-estate, easy to build upwards, fairly consistent weather, and lots of access to water. While these cities are often depicted as being laid out in a similar way as surface cities, this seems like a poor choice for defense. Walls and gates are trivial to swim over, and bombardment from above is a constant threat. Furthermore, traditional means of both ranged defense and attack may be useless underwater, given that projectiles can't be fired as effectively through water.

My merpeople are unsatisfied with this arrangement, and have commissioned their finest mer-architects to build them a more defensible mer-city. Their mer-king has approved the plans and the mer-chants have accumulated the money to build. What kind of threats do the architects need to keep in mind, and what sort of fortifications and defenses should they build to keep the city safe?

The merpeople in this world have fish-like lower bodies and humanoid upper bodies. they've got primitive lungs that allow for limited activity outside of the water but breathe primarily using gills. Technologically, they aren't terribly advanced, perhaps at the same tech level as the late Roman empire. They've got some metalcraft, using undersea volcanoes for forges. They don't have much in terms of explosives or gunpowder, though the mer-king is willing to invest in researching such things if they're the best bet for defenses. They have had significant success domesticating other undersea creatures, so sharks and whales are available as resources.

The king is primarily worried about attack from other merpeople, as well as some other unsavory underwater humanoids. He's got trade and contact with the surface folk, who are about at the same tech level as his kingdom. They fight, occasionally, but the combat is mostly restricted to the surface of the ocean, leaving the underwater cities untouched.

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    $\begingroup$ Can you elaborate on what common threats there are? Are we talking mer on mer action? $\endgroup$
    – James
    Commented May 19, 2015 at 18:18
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    $\begingroup$ I assume by bombardment from above you mean from humans, but what tech level are the humans at. Are these bombardments just rocks, or explosive depth charges? $\endgroup$
    – newton1212
    Commented May 19, 2015 at 18:19
  • $\begingroup$ Added some stuff on what the context of the undersea warfare would be and what tech level the merpeople are at. $\endgroup$
    – ckersch
    Commented May 19, 2015 at 18:35
  • $\begingroup$ Are you thinking following this question up by asking what mer-battles would be like? Because now I really want to know as much as possible about mer-battles (weapons to use, formations and maneuvers, defensible locations, essentially 'Art of War' underwater) $\endgroup$ Commented May 19, 2015 at 18:51
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    $\begingroup$ Simply locate your "atlantis" inside colossal coral reef ranges/mazes and say the inhabitants can move comfortably among vast poisonous sea anemones like the clownfish. Then from inspiration from Vietnam War there are miles of narrow trenches not to be confused with Mariana trench lol, also strong deep sea current make it inaccessible except yr Merpeople evolved powerful tails etc. Lastly naval mines... $\endgroup$
    – user6760
    Commented May 20, 2015 at 0:31

9 Answers 9


Make open water access difficult.

If the merpeople need to breath air, then what they need is a very defensible surface entrance to their city.

enter image description here

A deep cave with a single entrance near the surface allows for simple surface defences while maintaining underwater access. With the entrance being surrounded by a reef or an atoll, any invaders must wade through shallow water in order to approach the entrance.

Build the city inside a cave.

Being deep underwater, sunlight is already not a typical luxury for the Merpeople. Lack of sunlight being the main disadvantage of living inside a cave, the cave dwelling Merpeople will not suffer much.

enter image description here

The cave have similar benefits as a cave on land. It protects from all directions and allows for focusing defenses on the entrance of the cave.

If no cave is forthcoming, a deep trench with a constructed roof will provide a similar benefit. The roof could be grown from coral, which has long been used for construction.

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    $\begingroup$ Construction, Coral. Construction! Dad, stop. $\endgroup$
    – corsiKa
    Commented May 20, 2015 at 15:04
  • $\begingroup$ @corsiKa I am not familiar with, what I assume is, that popular culture reference... $\endgroup$
    – Samuel
    Commented May 20, 2015 at 19:38
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    $\begingroup$ Oh, uhh.. umm... cough knowyourmeme.com/memes/carl $\endgroup$
    – corsiKa
    Commented May 20, 2015 at 20:28


Simply enough your mer-folk have a rich number of options for appropriate military netting.

Net walls perform some degree of passive work of course but still suffer from the 360 degrees of movement in water. However trap nets could be designed with ballast (anti-ballast?) to deploy or be launched from the city; these nets could entangle enemies and the floats (anti-ballast I guess) would drag them to the surface.

Additionally whale/shark riders could work in teams to net enemies and then cut the nets loose from their mounts (whose strength overcomes the floats) again dragging their enemies to the surface.

Depending on the materials used (we have merfolk so I'm guess some fantasy is involved) the netting materials could be rather exotic like "Tempered Jelly Fish Tentacle Nets" very strong and causes pain to those caught in the net (preventing easy escape with something like a diving knife).

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    $\begingroup$ Jellyfish nets sound like fun $\endgroup$
    – Rob Watts
    Commented May 19, 2015 at 21:51
  • $\begingroup$ Mounts wouldn't have to work against the floats if they get their buoyancy from compressed air that gets decompressed during deployment. This also has the benefit of surprising the target with the net's buoyancy, which they might assume would merely entangle them if they couldn't see any flotation devices attached to them. $\endgroup$
    – talrnu
    Commented May 20, 2015 at 16:38
  • $\begingroup$ @talrnu I avoided the concept of compressed air (otherwise torpedos or propelled harpoons were options as well) since the overall tech level sounded to be roughly bronze to iron age levels. However it would definitely improve the action/deployment of the nets. $\endgroup$
    – Culyx
    Commented May 20, 2015 at 17:12
  • $\begingroup$ @Culyx True, though the compression and decompression of air or some other gas could be afforded by cultivation of some fictional (or "undiscovered") sea creature or plant if you really want the mechanism without the technology. $\endgroup$
    – talrnu
    Commented May 20, 2015 at 17:39

Since the mer city is effectively under threat from all angles, a dome is the most likely measure of protection. (If creatures or high tech opponents have the ability to tunnel, then the enclosure should be a sphere).

The dome provides direct physical protection against threats ranging from depth charges dropped from ships on the surface to torpedoes launched from around the perimeter. With small and protected access ports, the threat from enemy commandos (either merpeople or human divers) can also be reduced.

The dome also provides a naturally strong structure to resist overpressure from explosive devices, and the curved surface can reflect shock waves, to a certain extent. As a bonus, this uses the minimal surface area to enclose the maximum volume, something the treasury should be pleased with when looking at the bill.

The bill itself will be huge since the dome must be immensely strong. Even though it is supported by the volume of water enclosed within, the effects of shock waves and explosions will be hugely amplified by the physical properties of water (at 800 x denser than air and being incompressible, shock waves will move much faster and will not be attenuated like they would be in air). The deeper you are, the more static pressure is on the dome, to begin with, so the pressure differentials from explosions outside the dome will also be hugely amplified. The primary threat to the dome is pressure, so materials which have high compressive strength must be chosen (materials with high tensile strength like Kevlar or Graphine would require some very unusual building techniques to effectively use their strength).

The dome will also have to be protected from the environment, just as structures on land are. The RMS Titanic is already decaying due to the presence of bacteria which "eat" the iron of the ship, so the outer surface of a metallic dome would need to be coated with some impermeable, non reactive substance to stop the effects of bacterial decay or just ordinary corrosion due to being immersed in salt water. The King of the merpeople will be annoyed at the amount of gold needed to cover the dome...

Other possible substances to build the dome out of could include concrete, diamond or perhaps more simply using electrical current running through a wire armature outlining the shape of the proposed dome to cause minerals to precipitate out of the sea water and form a natural sort of "concrete". This is similar in principle to how some sea creatures accumulate shells, and may be a better "fit" for how the mer people actually build things. A dome built in this fashion will be much cheaper, but not as strong as an engineered dome built out of "our" sorts of materials.

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    $\begingroup$ So how could they breathe? Like any marine mammals they need frequent access to the surface. $\endgroup$
    – JDługosz
    Commented May 19, 2015 at 21:15
  • $\begingroup$ Why would there be more static pressure? If they're already living at that depth, they don't need to dome to be pressurized. $\endgroup$
    – Rob Watts
    Commented May 19, 2015 at 21:50
  • $\begingroup$ Monel, marine bronze, or another such relatively passive copper alloy would be a good choice for protecting a conventionally constructed dome; not only do such alloys resist saltwater corrosion well, they have good antifouling properties, which keeps biological attack at bay. $\endgroup$
    – Shalvenay
    Commented May 19, 2015 at 22:32
  • $\begingroup$ You might also consider a net-based dome, which would simplify construction greatly. Think a giant buckyball. Simple folk could use something like reinforced or coated kelp, while advanced fortifications could use strictly solid construction materials as you suggest. The style might be a bit like a cage, but it could also have gates at various places - while at the same time the spaces could be designed only big enough to keep out the primary threat (so regular fish could come in or out, or even people, but not big monsters, shark riders, etc. $\endgroup$
    – BrianH
    Commented May 20, 2015 at 16:32
  • $\begingroup$ I am assuming mer people use gills like fish, so as ing as there is a sufficient flow of oxygenated water maintained inside the dome, the mer people will be fine. $\endgroup$
    – Thucydides
    Commented May 20, 2015 at 20:45

Domes seem obvious but also expensive and perhaps a bit technologically beyond your mer-people. So I am going to break this down by threats and how to counter each.


Artillery in the underwater realm would best work if it is literally just dropped from above...the resistance the water creates would make launching it in a conventional matter ineffective as the range would be very very short.

So. You have boulders dropping from the sky...err...upper ocean I guess. Diverting is the best option here. I would suggest attaching strong slanted covers to the to the tallest structures in the city. The structures should funnel the stones or whatever is being dropped into certain locations within the city or outside the city perimeter (in a large city you probably need to do both). Just make sure people know where not to be during the fighting...squish.

Anti Personnel

Here is where you can get real nasty on defense. Create nets made of concertina wire. This is essentially barbed wire...with razor blades attached instead of barbs. Depending on visibility you can combine that with stirring up sand and grime from the bottom to make it harder to see...or camouflage it with growing sea grasses.


Boats: boats/submarine are going to be tough to stop...if strong enough they could break the razor wire nets...but the wire could also tangle propeller propulsion. Again...in a 3D environment I think nets are going to be a good bet. For boats you could simply create big chain nets to disable them.

Tanks: Just because you can attack from above doesn't mean ground based attacks are worthless. You can still have tanks (think battering rams with the roof) along on the ground so trenches and walls and barricades will still have their place they just won't be quite as primary.


Keep in mind the enemies gate is down (relevant username).

The principles which apply from Ender's Game are quite relevant here. Single hardpoints are very difficult to use to defend an empire when you have multiple dimensions. If the king has territory which is also needing defense, any of these can be bypassed.

You cannot defend something easily from 3-directions. This might seem obvious, but is the fundamental assumption on which all defensive systems must be understood for a city which can be easily attacked from most directions (not below, unless the city is floating).

So the first thing the king needs to determine are:

  • Can a singular fortification be the entirety of defended territory?
  • How long can the city sustain itself if under prolonged attack?

You might take advantage of existing natural geographical structures. Another answer addresses caves, but keep in mind underground mountains/valleys are also much more defensible - what you want to do when using these is dictate how your enemy approaches. Shallower areas will help avoid the angle of attack from above by allowing you to "wall" the distance from floor to ocean surface.

Areas along shoreline with steep dropoffs provide considerable defense against other primarily underwater enemies.

Keep in mind, in some sense, the more defensible it is, the easier it is to isolate and place under siege.

Next, determine whether having an early warning network is beneficial or feasible. There are lots of whales which can communicate vast distances underwater which would serve as a patrol network. Keep in mind the further you go away from your city the number of patrols required to have the same density goes up nonlinearly.

You also want to have a very mobile army to respond to attacks. Since they can come from many directions easily a military which can be easily mobilized will be a huge asset.


Not sure if you can easily erect defensive structures that would be too effective here...most things can easily be swam over/around and don't provide much for cover. So I'm not sure if fortifying an underwater town is a feasible venture (a dome is in other answers, but I believe this is far too expensive to build from scratch and potentially far outside of their technical abilities). I believe you are going to be in a 2 phase defensive approach instead.

First phase - The simplest defence of this city is keeping it hidden. A network of forts on the sea floor along with floating 'forts' suspended in the waters will help them keep it this way. Simple idea...the mer-city is surrounded by forts (or whatever they might be) that detect and intercept any incoming creature that shouldn't be there. Keeping small yet fast strike teams (shark use works here) that move to intercept any invader so they cannot locate the city is important. Traps, barbwires, nets, and any other deterrent these mer-folk can come up with can be employed here.

The forts need to be positioned so they have good lines of sight and only have one or two entrances (bottleneck defence, a narrow opening defended by a series of tridents would not be easy to push through). They need to be far enough away from the city that they are out of sight, but still be close enough for the guards to travel to and be able to communicate back to the city quickly.

Second phase - A castle. Your network of forts are going to intercept most small threats, but a larger 'invading' force will be able to overpower it. When the forts (or scouts) detect an incoming force that can overpower the forts, the alarm in the city is raised and people get out of the city and into their castle. Kinda the 'helms deep' approach to defence...a castle well stocked with supplies can be a place to hide and wait out the attackers.

Castle is best as underwater caves (natural or dug out by the merfolk) with narrow tunnel entrances that can either be defended with a series of tridents, or outright blocked off from the inside (a large bolder that normally sits out of the way can be pushed from the inside and block off the tunnel. It's got to be big enough to hide away the population of the city and hold enough supplies to keep them alive for a year or so. Tubes with 'straws' can reach up to the ocean floors from the caves to keep fresh water (and oxygen) flowing through the 'castle caves'.


Some ideas for defending the mercity against attack that do not involve fixed, solid fortifications.

  1. Black smoker. These undersea chimneys expel dense clouds of mineral and sulfide laden water. Looking down from above one would see only opaque clouds of white or black. Additionally the sulfides and chemical content of the expelled water would be tough on water breathers. I could imagine capped smokers which could be uncapped in emergencies to release plumes of toxic hot water.

  2. Oil well. The merpeople could have oil wells which they open during battle. This would also contaminate the water column above the city, but not as much as the soluble sulfides from a smoker. An oil well would be useful against attack from above - after the surface was slicked with oil, floating "bombs" could be released to ignite it. This would be good against boats. It would be even better against things which must surface to breathe. It is not specified if the merpeople in this scenario breathe air or if they have gills. If they breathe air and have a supply in the city, a fixed surface fire fed from below would make it very difficult for air-breathing aquatic attackers based at the surface.

  3. Clathrate / natural gas / air bubbles. Clathrates are solid forms of methane ice which can degenerate into gas with heat or shock. A column of bubbles would be a fine defense against aquatic attack from above. Bubbles occlude vision and also sonar, and dampen sound transmission. A thick enough cloud of bubbles reduces the density of the water column. A dense column of bubbles intercepting a swimmer would cause the swimmer to fall through it. Some people think the Bermuda Triangle phenomenon is caused by dense clouds of methane bubbles which come up under ships, causing them to fall into the sea. A fixed source of bubbles could hide the city all the time.

  4. Kelp. Siting your city in a kelp forest means it cannot be in the abyss. But 150 feet is deep enough for a respectable city. A kelp forest city would look extremely, extremely cool. Kelp is not very dangerous itself. But you cannot see through it or sonar through it. Attackers would be at a disadvantage in that defensive measures (see #7) will be all over them before they know what is going on.

  5. Currents.. Siting the city beneath a strong current would be fine protection. Attackers sallying down from the surface would be swept sideways by the current; likewise bombs. There are places where underwater topography channels water movement to produce currents of this sort. Perhaps the merpeople construct such or augment existing natural features. A strong lateral water current would be more penetrable than a wall, but you cannot batter a hole through a water current. A water current of this type would sweep away attacks intended to poison the water. Bonus: if you pee while defending your city no-one will know.

  6. Jellyfish. Over the city loom five colossal and many, many smaller jellyfish, their tentacles hanging down. These creatures are almost impossible to kill. Even dead and in pieces, their tissues continue to sting. They are fed by tenders wearing special suits and so converge hungrily on anything which gets close to them. Any attacker traversing them must also wear a protective suit, which looks really stupid. Prince Namor types in their scale speedos will definitely go down at a distance and come in laterally.

  7. Octopi. How can you have a merperson fortress without trained octopi? These defenders sally out from their holes and use their 40-foot tentacles to seize mounted attackers approaching laterally. Oh, you think your sword will slice those tentacles off, do you Mr. Meragorn? It turns out chain mail flexes wonderfully with the tentacles and octopi are so strong the added weight means nothing. Hmm. I see the attackers have brought their anti-octopus measures...


What I naturally think of is to build the city into a coral or rock wall, the same as if humans build their city onto, or into, a mountain side.

If possible, build it into a wall as close to the surface of the water as you can. Even better if the wall has a concave shape where the city is. The city mer-people get plenty of light (possibly making it hard for the deep mer-people's eyes to adjust) The attackers may also have to let their body adjust to the elevation if they usually live very deep. Also, the city is now naturally defended from enemy mer-people by the water surface above and the rock wall to the rear, leaving forward and below to defend.

At first I was just thinking houses and buildings on the face of the wall, but it might also be possible to find a cave system with limited entrances (like a hole), or have the city within a large crack.


Inspired by DoubleDouble's idea of 'build it as close to the surface as you can' to give little up-down manoeuvring and so the defenders have the advantage of being used to lower pressures and higher light levels:

Build it only a couple of storeys below the water surface - the walls only need to reach up as far as the surface is - a dome is not necessary as the merpeople (I assume) cannot breathe air and are unlikely to be able to leave the water to go over the walls.

Alternatively, build it in a very large lake a couple of miles from the ocean. This lake is only accessible via an underwater stream, the entrances and exits of which are heavily guarded, and maybe a few emergency exits that lead from the lake to a couple of meters away from the ocean (never live in a place with only one exit).


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