What would a modern fortified castle look like?

Question

Assume that paramilitary siege weapons were proliferated to the general public after America fell. Details below, but basically short of multi-billion dollar tactical weapons such as advanced fighter jets and nuclear submarines, anything that can be built by a resourceful family in a well-equipped garage is available to the Feudal Lords of America. You want your dynasty safe when Uncle Sam can’t help you any more. What do we build?

Weapons

I will set a weapons price cap at $500,000 in US equivalency, 2020 dollars. This price is not only the weapon but includes the research and production facility to design and test and make it. The cost estimate is not expecting hard science.

The premise here is based on the ROI: No siege victory will recover a cost greater than this in a reasonable time, and investing long-term is stupid when your neighbor may lay siege on you tomorrow just as well. A castle is designed to defend against local turf wars.

Examples: Per my limited knowledge, a neighbor may lay siege with weapons from the following list, which is ordered by availability from cheapest to most expensive:

• Sniper rifles,
• Assault rifles,
• mortars,
• flash-bangs,
• bazookas,
• fast rappelling gear,
• thermite,
• canons,
• RPG’s,
• artillery guns,
• armored tanks (primitive ballistic),
• catapults,
• private aircraft with bombs,
• dirty bombs,
• EMP weapons,
• chemical bombs,
• cyber warfare viruses (controlling or disabling fortification systems)

Infrastructure

Metal and ore processing is available to most Lords at a high cost. Assume they have access to energy (which is what makes them Lords), so they can forge and shape metals and machinery. Manufacture a 6-ton canon will take a lot of metal away from something else. Having more than one tank is very unlikely. There is no federally regulated manufacturing infrastructure. Assume you are not the unfortunate neighbor of an iron mining dynasty, but you can trade with one as well as anyone else.

No first-world America weapons are left. They were used or have become unusable through neglect. One “may” renovate a tank, for example, but it has no support structure building custom parts or lubricants or artillery, so it is of little more use than a large rolling mass. A tank would be more effectively built anew.

Given: Castles of old were conceived very specifically to combat the weapons of that age. Battlements, towers, arrow slits, burning oil channels, motes, draw bridges; all engineered for weapons of that age. None of these features generally apply to modern siege weapons.

Castle Materials

The typical castle has ready access to stone, concrete, brick, wood, and earth. Metals and ores are costly and need to balance between offensive and defensive applications. I.e., a dynasty building a fully armored assault regimen likely has a picket fence around their property, and vice versa. Build a tank and likely you have three less tractors working your fields.

The fort

The fortification would be a routine safe seat of government as well as an emergency shelter, it would not normally wall off the whole community which may number as much as 25,000. It should provide short term emergency shelter for a siege of several weeks if needed, allowing time to secure outside help. So stores and warehousing materials to rebuild are part of the function.

The adversary

It is peace time however there is no rapid response from the government if your property is attacked. The public generally disfavors war because everyone knows they could just as likely be next. So, castles plan for small paramilitary forces operating covertly or in a clandestine assault under direction of some Lord. A force would be expected to be no larger than 75 soldiers.

What would a castle designed to repel modern paramilitary siege weapons look like?

---

Effectiveness: The solution needs to be a deterrent, given that an impenetrable fortress can’t exist. A castle simply makes siege “not worth the expense.” Every dynasty still needs to meter the amount of pissing off they do. That’s my job as the story teller. Political correctness and all that, you know.

Answer 1

Historically, fortresses mostly died out after WWII. Partly because the Maginot line gave them a bad reputation, even if it worked as it was supposed to -- blocking some obvious German and Italian avenues of approach, and allowing the mobile forces to concentrate elsewhere. Don't blame the line when that elsewhere was the wrong elsewhere. And partly because what happened to German fortifications after the Allies controlled the air.

Fortunately, even a well-equipped garage won't do four-engined prop bombers or 420mm siege guns. The boring machinery for modern heavy artillery barrels is quite specialized, and so is the rifling. Even making a Dahlgren would challenge your attackers with the sheer amount of steel. And look at the size of a WWII aircraft factory. WWI aircraft were much more feeble. So the things which killed the historical fort are going away.

Metal being on the shortages list puts a spanner into the works, but there were historical forts with non-rebar concrete. And concrete is on the ready access list. (That's a bit strange, BTW, producing lots of cement is a major industrial undertaking.)

So take a look at the Brialmont forts. On average, some 50,000 cubic metres of concrete, which would come to $10M or thereabouts in current prices if there is no discount for quantity. This would be proof against anything the 'garage workshop' standard can throw at it.

Answer 2

Islands.

https://mainehomedesign.com/bicentennial/fort-gorges/

In a fallen but heavily armed US, roads will be impassably dangerous. Water travel will once again be important and so strong points must control waterways. As regards withstanding sieges, those wishing to attack an island fort must either have weapons able to reach the fort from the nearest land or be willing to approach by vulnerable boat.

The other advantage as regards a fiction is that most islands suitable for forts already have forts on them. Most of these forts are over a century old. Using an existing old structure in a fiction gives a launching point for creativity - imagine the persons taking over Fort Gorges (depicted) and what they would need to resist attack from the shore. Could you restore Fort Gorges to its original working situation with local materials? What would you add - maybe a spotting tower with a sniper? Working with and around real things makes a fiction more plausible.

Answer 3

The real defense is not a physical defense, but the people.

When the Lords have the support of the people, they do not need to build a castle designed to force their will upon the local population.

Most castles built since the 1500's were built not to withstand a siege, but to impress the neighbors through ostentatious displays of wealth. Many human beings worship wealth and many have the belief that the wealthy person should be supported by being taxed. (And some even believe that a wealthy person will work for the good of all.)

Once the local population supports the castle, it becomes the seat of local government, the place where people go to get justice, and where regulations and permits are handled.

When the castle is where many people get some kind of justice, then the real defense against a mob trying to siege the castle is actually the people who live around the castle and will defend it to defend their way of life. So, if a neighboring country tries to pull together a massive army to invade and overthrow the Lords, the population will fight to the death to resist that invasion.

In the case of a small terrorist band sneaking in to attack, that is more a police action rather than an army action. Instead of trying to prevent, it is better to have fast response teams that can surround and deal with the small group.

Resilience is a better deterrence than meeting force directly.

Answer 4

I love this setting. A bit like the colony (2010) reality tv show, and some parts of the walking dead, where they build the base. I always wanted shows to be more about this base building and less about just scavenging and fighting.

I would define goal as how to build a base spending the least resources, that would allow to defend against an opponent with the most resources possible. This way most opponents will not be interested in attacking, either because of the high cost of the attack itself, or low cost of the stored goods in comparison.

Concrete is of course better than sand, but it is also much more costly, especially if you consider complexity of getting it done, grinded and fired, after all the big machines are left far away or destroyed, and transportation cost becomes high. Metal is likely also to be too costly for the main structure, it is much better be used for tools, to pay for some active defences. So main part of the structure I would say to be made of sand, in particular sand bags. The bags themself can be mass produced even in low tech society, as long as fabric and sewing machine is present. They are also stored en mass in military bases, for this exact goal, fortifications.

Cloth itself, even plastic, will degrade under the UV, so the walls will have to be covered in paint or plaster or even dirt, and regularly updated. Not just for the looks, broken bags will not hold the sand as well.

Sandbags can be used to build several stories buildings too. Floor is to be made of timber, as only it can hold this type of load that floor needs.

Sand bags are also good against weather - wont be blown away like steel sheets. Good against fire weapons - even if bags will burn, the bulk, sand, wont burn. It is much more bullet proof than plywood or thin steel sheet. It provides cover against explosions, unlike lighter steel sheets that will be blown away. Sand bags are also not rusting and cheap to cover or replace. And even in case of earthquake sandbags are easy to put back in a wall. Sandbags are also used against flooding - they prevent the water from flowing, unlike almost any other solution. Closest alternative would be trenches, or lower-than-ground structures in general. But those are damp and require about 10 times as much work to do. Sandbag structure can be comfortable and dry, and even have much larger span, where trench-like structure would require massive excavation. Stones could be used in a similar manner as sandbags, but they require much more work to fit them, either with stone chipping, concrete filling or time consuming stone selection. Neither is anywhere as cheap, while the end result is similar. Stones are also much more rare to find, so delivery distance will be greater, which is bad, especially in hostile territory. Sandbag structure can be expanded from locally sourced material almost anywhere on the planet.

So far this gives protection from small firearms, fire weapons and small explosives, gives good firing position, allows to build massive structure even with small workforce, quickly and cheaply. Next level of threat is a perimeter breach, using trained troops at night or armored car, both are much more expensive.

For this cheapness of sandbags plays a major role again. Bunker defence idea goes like this: long corridors are ending with a gunspot, in a way that to reach the gun area, people must walk down the long corridor. Same works with sandbags labyrinth, even if there is no ceiling in that area. Main idea is to make all the defence directed one way - so that whenever people move from outside to inside - they are always seen. But when people walk from inside to outside - less so. This also simplifies detection - less places to put signal traps. And helps with the car - car wont be able to ram through many sandbags. And if people will try to just climb the bags, they can be seen from the central structure that is taller. Bunker defence type of labyrinth may seem expensive, but it gives just unthinkable advantage to the defenders. Just a few people can stop hundreds of attackers. And labyrinth can be used for storage, less expensive items from outside. Labyrinth made of walls going outside in zigzag, rather than normal walls that simply surround protected area.

At this point only heavier equipment can provide any sort of advantage. In particular high pressure flamethrower and mortar - they are effective against light bunkers and such structure.

Next level is to make free area all around the labyrinth to provide line of sight. Remove tall plants, ideally leave empty soil to remove any chance of cover for the incoming forces. This is where sandbags are useful again - the process of filling them in this area provides this territory without vegetation. Flamethrower, even high pressure one, needs to come closer, and lack of area to hide makes that much harder. With mortar it is a different story. It can fire further than the line of sight, and there is nothing that you can do about it. Labyrinth provides good protection statistically, chances to be close to a place that a mine will hit is low. And sandbags provide a good protection from shrapnel. If opponent is using modern mines, with proximity sensor, that explode several meters above the target, some sort of ceiling is needed. Solution in general is to use mortar of your own. Or if not, just wait. It will takes many tons of mines to make sure that such a labyrinth will stop defending. In particular gunspots at the end of straight paths could be used as reinforced spot, with ceiling and added protection, to make sure that not even infantry advancement during the borbardment will help.

Next level is siege. It cost time, and that is the most costly thing. To make sure that you are fine with it, make sure to have as much supplies as possible. And a large labyrinth helps with it. In particular rain collectors and solar pannels, all require area, and being able to protect this area makes required siege time almost impossibly long.

Next level is chemical weapons, forest fire, river flooding the whole area. They are especially good against bunkers, as those elements often heavier than air and go down. Not having a lower-than-ground structure helps a lot here. Having large area free of vegetation helps - it prevents forest fire from coming closer, it makes wind stronger, allowing smoke and gas to disperse quicker. In particular curdistan cooling tower can help a lot - provide cooling in a hot day, and also help with removing toxic gases much faster than any bunker ventilation could. Sure, fancy bunker ventilation can be better if it is really scaled up, but cost will be extreme. And against flooding sandbags are placed in entrances - allowing water to rise higher than the floor level, but not letting the water in, in extreme flooding people may take refuge in a central rower that is higher. Labyrinth of sandbags will make sure that at least dangerous surge of water is broken down into softer water increase. This way even in extreme case, like broken dam, the labyrinth will get partially destroyed, but not the central tower.

For any more significant threat this settlement will require much more people. And I would advice to have as much population united as possible, as this allows for even cooler defences.

Expenses: sewing machine and people knowing how to use it, parts and oil (1000 usd), generator and lots of fuel (1000), lots of fabric for bags and rain collector (1000), a few guns (1000), solar panels and batteries (1000). And a few months of work to fill the bags and put them in place.

Answer 5

Underground Fort

We've already seen what "modern war on the cheap" looks like - in Syria it looked like dropping IEDs out of helicopters, and in Ukraine it looks like long range dumb bombs (missiles, rockets, and artillery). In both cases, these weapons are used to indiscriminately bomb city centers, and appear to destroy even built up concrete structures pretty effectively.

So the defenders go underground - and without specialized "bunker buster" bombs, they are significantly safer.

Planned vs Ad-hoc

For the most part, the warren of tunnels that sprout up under modern battlefields are ad-hoc: the tunnels came long after the city itself.

It's possible to build the tunnels first, and then the city. This might let you do things like build mini-forts above ground, where you can control choke points, or rapidly deploy anti-air weapons, or... any of innumerable different tactics you can think of once you go down this road.

The key is starting with the fort: assume you're going to build multiple, interconnected underground structures, and a warren of tunnels connecting them. Now design the rest of the city.

Answer 6

500k is not enough.

M16s used to be sold to the government for about $650, the M27 that I was issued cost the government 1,300. I have no idea how much the new Sig rifles cost, but Infantry rifles are the modern spear. It's the foundation of your fighting force.

According to a 0341 Sgt that I pestered with too many questions a while ago, the ENTIRE mortar system with all of its extra toys costs millions of dollars. The tubes themselves are worth 200k (81mm M252 mortar). M240s cost just under 7k, an M2 used to cost 1200 during WW2 (I do not know if that is adjusted for inflation or not) but the Army was paying 12k for them in 2012.

The point is if you want a properly equipped infantry company, just under 200 dudes, you are already going to be over budget. And a company is the smallest your force could be and still function in this role.

Sigfried line or Ostwall

You don't have to worry about MBTs but tanks and other improvised armored vehicles are still a problem. An M2 might do the trick but not alone and only on lighter vehicles. You'd need Dragons teeth and AT (Anti Tank) ditches. And A LOT of them. Fill in the gaps with anti-personnel, AT mines/IEDs, and a healthy serving of barbed wire and you have a decent outer perimeter.

For the inside where your defenders will fight, there's the 'survivability onion,' i.e. the best way to survive an engagement with the enemy is to not be seen in the first place. A trench network reinforced with concrete in key positions and extensively camouflaged. If built properly, only enemy indirect fire would be effective against your position.

Placing machine guns on positions with roads, river beds, or any other form of terrain that an enemy force would use to approach. If you have AT weapons like RPGs or LAWs, those would be in positions near the machine guns but not the same position. This prevents you from losing all your firepower to one lucky mortar or artillery hit. The mortars would sit in camouflaged firing pits near the 'keep'.

My personal experience says to not tie down your defense to one position but to remain mobile. Moving at least every 24 hours launching patrols to harass the enemy, or gain recon for an envelopment maneuver of the smaller invading force.

Answer 7

1.Underground, put layers of earth and stone natural armor in your defense.

2. Reforced concrete, manmade armor works more effectively if your base is underground.

3. Missile defense systems and anti aircraft guns.

4. Air force, and artillery. Some times the best defense is offensive. If your airforce destroies thiers then it can't penetrate your defenses.

Answer 8

Note that the fort mostly disappeared with the advent of gunpowder. There's a reason for that: It's cheaper to blow up fortifications than to build them.

Fortification got a limited reprieve in the form of the pillbox due to the difficulties in transporting the firepower needed to reduce it.