What would a unit built purely for fighting in extremely large buildings for look like? The biggest thing about this is that these buildings are so large that anything not carried with the forces in an engagement can't be used.

These buildings are at the size where for the vast majority of the fighting takes place without anything out of line of sight. These buildings are primarily internally connected by narrow gauge rail networks. Although there are areas where the corridors could be driven through using a large car. There are rarely very large rooms in these buildings up to a size of ~5 hectares, but these are rare & most are to 1-3 hectares. Most of the ceilings in these buildings are 3.75m (12'4 feet) tall. Genetically modified giants that are 2,4-3,2 meter(8-10'5 feet) tall & suits of power armour are available. But the power armour is expensive. Not everyone is a giant or can be in power armour. These units can have equipment built primarily for their use. There are other units that fight in more conventional types of warfare. These units aren't the entirety of the military & there are about ~90,000 people in a service dedicated to this type of warfare. Civilians aren't of concern as there are none in these buildings. There is a near future level of technology available.

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    $\begingroup$ Why do you use superior meters for ceiling height but switch to imperial for the height of the power suits/giants? $\endgroup$
    – Demigan
    Aug 25, 2021 at 6:28
  • $\begingroup$ @Demigan i use metric for everything except person heights. my brain finds it easier to do imperial for people heights. but for everything else imperial is incomprehensible & hurts my brain. $\endgroup$
    – OT-64 SKOT
    Aug 25, 2021 at 7:02
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    $\begingroup$ 5km^2 squared rooms and "nothing is out of line of sight" doesn't compute. Your rooms would be like a forest with all the pillars needed for support. $\endgroup$
    – Abigail
    Aug 25, 2021 at 15:39
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    $\begingroup$ Questions: are the buildings tall as well, that is, does fighting happen on multiple floors? Should the buildings survive the fighting, or is blowing up (parts of) the buildings all in the game? $\endgroup$
    – Abigail
    Aug 25, 2021 at 15:42
  • $\begingroup$ Are those 5km^2 squared rooms having the same 3.75m ceilings? $\endgroup$
    – Alexander
    Aug 25, 2021 at 17:26

9 Answers 9


Your unit needs to have a lot of expertise in many areas

The constraints would determine what is optimal:

  • No air support in the sense of bombers or fighters, but you could still achieve combined arms with 'very' close air support - perhaps small portable drones or UAV's if ceiling height is not too low
  • Not too heavy mechanised units - floors are always designed only withstand a certain amount of weight. So no heavy tanks (or even medium tanks) as these weigh tonnes. You can only really have light weight options, such as tankettes or LAV's (even then it is risky)
  • for such wide dimensions, you still need to transport supplies and support to your units. So a light weight transport is essential, perhaps a lightweight car (similar to those used by Special Forces to be dropped behind enemy lines and assembled / disassembled)
  • Your units themselves would need to be trained in urban warfare and room to room combat. This is mostly close quarters, however as you mentioned large rooms too you would need them trained in combat at range too. So perhaps relatively manoeuvrable small caliber light arms (such as a P90), lots of grenades, but also equipped with long range rifles and sniper rifles too for large spaces.
  • Keeping up all these floors, roofs and building elements requires structure. You need an engineering unit to ensure that your force is not defeated simply by demolishing structure.
  • Communication, depending on wall types etc. and command is also an issue and needs specialising. You need commanders that know more about building plans rather than terrain.
  • You need intelligence units that know how to communicate through structure, and have computer / manual systems able to establish and monitor where enemies and friendlies are. This can get complicated, and can determine success, so do not discount this.

Actually, you may find once all of these are involved, the building may not last long at all, no matter its size. Once combat occurs, destruction pretty much turns up everywhere.

  • $\begingroup$ "actually, you may find once all of these are involved, the building may not last long at all, no matter its size. Once combat occurs, destruction pretty much turns up everywhere" the thing is these buildings are so big that it would be like trying to destroy a mountain. $\endgroup$
    – OT-64 SKOT
    Aug 25, 2021 at 5:33
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    $\begingroup$ Good answer, a few additional points: Don't forget gas detection / protection - much more vulnerable inside buildings where gases can't just disperse. By the same token, assuming that the "normal" building power gets taken out early in fighting, need to bring in generators / solar panels to power own equipment, especially ventilation and winches for moving loads between levels. And OT-64 SKOT - no matter how big the building is, modern weapons will destroy large sections very quickly, it will not have unlimited structural redundancy. $\endgroup$ Aug 25, 2021 at 6:02
  • $\begingroup$ Why on earth would you use a P90 when it is mostly ignored by todays units and a carabine is a perfectly viable, and used in real life!, option? $\endgroup$
    – Bartors
    Aug 25, 2021 at 8:07
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    $\begingroup$ What are your objectives? Are you trying to conquer a hostile population, rescue prisoners, genocide, capture territory, capture information ...? Those will affect what weapons and tactics you use. $\endgroup$ Aug 25, 2021 at 15:52
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    $\begingroup$ @OT-64SKOT - "the thing is these buildings are so big that it would be like trying to destroy a mountain" I'm not sure that's an apt analogy. If a mountain was mostly hollow, like a large building full of large rooms, then it would probably be much easier to destroy. It's not the size alone that makes a mountain difficult to destroy, it's the size coupled with the density... $\endgroup$ Aug 25, 2021 at 18:18

"Sir!" Said a soldier, running up to where a group of squad leaders were planning the days action, "We've got a problem"
"What is it Rick?" Asked one of the leaders.
"The enemy have gone underground, or, well, kind-of." Rick paused to gather his thoughts. "You know those big structures on the orbital scans. We thought they were some sort of monument: solid rock like the pyramids on Old Earth, only bigger." The squad leader gave a brief nod, and Rick continued. As soon as we got here, the enemy rushed towards them. Kyle was on the scopes and said it looked like there were some doors in the sides of them."
"Get to the point Rick"
"They're hollow, like caves above ground. We set some ground penetrating radar on the side of one and it looks like there are spaces up to kilometers in size in there"

The leader who had been speaking to Rick turned to face the others. "Can we call in an orbital strike?"
"Nah Josh, they're saving the munitions for the offensive on Telaun next week. We're also not authorized to use nukes down here either."

"Righto," said Squad Leader Josh, "I guess we'll be doing this the hard way. Rick, how big are those doors? Can we get the armour through them?"
"The power suits, yep. Maybe some of the smaller light vehicles too, but nothing like a tank."
"Rick, give me the data from the scans and go grab the nerds, we need to get those auto factories running ASAP"

Rick ran off, and shortly therafter the lead engineers arrived at the meeting.
"Up on the screen I've got a diagram of where we're going to be fighting. Mostly it's cramped. 3 meter ceilings, miles of tunnels. Occasional big rooms. We need a way to be able to fight effectively in there. The way I see it is that our main issues are logistics and navigation. We can't get any supply trucks in there, and our GPS units aren't going to work."
"There's another issue Josh," one of the nerds begain, "with that much mass around, communication via radio is going to be terrible."
"Right, Logan, that's your problem. Grab another nerd and work out how we can talk to each other. Anyone want to take on staying supplied?"
The nerds looked at each other. "Mack should do it, he knows the vehicle blueprints best".
"Ok, Katherine, you'll be supervising that, we need to be able to operate for maybe 3-4 day expeditions. See if we can't get some heavier guns in as well."
"I'll take navigation", said a bald engineer, "I got a couple ideas. I'll need to run some simulations on the ships mainframe for it though."
"Done, I'll make sure you get access"

What solutions do they come up with?

Radio just can't penetrate far enough. In a single corridoor or room it's fine, but 5-10m through a wall and the signal is toast. I've got one of the autofactories pumping out coils of wire and radio repeaters. We'll drag 'em along and dump a repeater every now and then. If we have a maze-like-bit it'll be better to use wire than a dozen repeaters. The data suggests there are rails set through those buildings. If we're lucky we can use those as signal paths as well by clamping wires on to them.
All our troops will still be using their normal radios, but any communication further than their immediate vicinity will go via the repeater network.

Mack + Katherine:
We can definitely get our LUV's into those tunnels. I've tasked two of the autofactories to making mods to them. Some we're weaponizing. We've up-armoured the bonnet, and mounted a 30mm autocannon in the passengers seat. It can be operated from a guy lying prone behind the cab on the flatbed. We're also putting together some small trailers, a couple for fuel, some for food/water an ammunition. Turning around will be tricky, so we've put hitches on both ends of the trailers, and towballs on both ends of the LUV's. This means we can turn around vehicles individually and hook them back up. If we need to, we can also push the trailers rather than have to reverse heaps. Make sure there's a spare LUV following the convoy.
We've also prepared some mining charges. If we need to we should be able to blast through into neigbouring tunnels.

Bald Engineer:
GPS is too high frequency to penetrate. Inertial navigation is fine for an hour or two, but will drift after that. We're going to be transmitting a single ELF pulse every 15 minutes from the ship. That can be picked up by the repeaters Logan's putting out, so they can take bearings to the ships location. By comparing the pulse receive time on a couple repeaters, the ships location relative to them can be determined, and because the ships orbit is known, we can figure out where the repeaters are. Each soldiers HMD gear positions itself relative to their nearest repeaters location, and uses inertial measurements when they can't get signal to a repeater. If a soldier is away from a repeater for more than a couple hours he'll be lucky to make it back - no communication, no navigation.
It isn't going to be particularly accurate. We'll be lucky to get 50 m precision , butI think it's the best we can do.

Dude, there are some awesome stories you can write in that setting.....


All robots.

robot sentry


These buildings will be occupied entirely by robots. The building will be filled with nerve gas and also smoke with metal particulates to block robot sensory modalities. Robots using active sensors (e.g. radar, sonar) will immediately be detected and fired upon. Small suicide bomber Roombas and dragonfly drones will zoom along the floor and ceiling up to anything that does not match the map of the area and blow it up.

The robots will stake out territories and set out defenses to disrupt movement along the floor, and then watch. Periodically robots might sally forth and try to capture more territory. It will be like World War 1 but inside, and all robots. The building becomes no mans land.

Your giants in exosuits will refuse to enter. They will be blind in there. The giants are tough but if the suit is disrupted they will die from the nerve gas. The giants especially hate the suicide Roombas.


I'd say start with everything an average military squad carries, including rations, and only omitting a few "outside use" items like tents and maybe pick some shorter weapons to allow for more mobility in the more narrow corridors.

Since near future tech is available, I'd suggest using equipment robots. Like those Boston dynamics is developing. They'd be better at moving up and down the stairwells and over debris. Longer distances could still be done with transport vehicles and the railway.

Stock up on extra explosive breaching charges and a battering ram to open the many doors you'll encounter.

Lastly, bulletproof shields. Cover is limited in buildings, especially in hallways. With power armour available those could be used to carry some mighty strong shields keeping both themselves and their teammates safe.

  • $\begingroup$ I particularly like the shields - suggests something like modern SWAT tactics. "Stay behind the Powersuits and shoot anything that doesn't say ARMY on it" $\endgroup$
    – ShellGhost
    Aug 25, 2021 at 14:48

Some equipment you will want to have:

  1. Smoke Machines: Smoke machines are used in order to force Enemies out of a small space or in order to find alternative exits to a room without entering. This way the enemy comes to you, and not the other way around.
  2. Dogs: Dogs are very useful in closed combat, because they can be used to detect and check for traps, sometimes used as fodder to trigger traps on purpose.
  3. Bomb squad robots: for scouting around dangerous corners.
  4. LAW missiles: These are used to attack through walls. It's a closed space, so your attack vectors are limited and predictable, unless you can attack through walls. They will also be useful against the giants.
  5. Some kind of kinetic projectile: For consistant armour piercing these are essential, but the guns usually used for these are tank guns, so it would be a large gun, but not a tank gun.
  6. lots of cable: Stone, concrete and iron are all really good at blocking radiation, making radio through very large building or in bunker difficult. laying down cable for long range communication would be nessecery.
  7. Lots of grenades: Grenades are used for clearing rooms in buildings. A grenade is thrown, and only then you enter. it should shock, and possibly injure anybody inside the room. if it's a big building, you'll need alot of them.
  8. bullpup rifles and pistols: reducing the spin radius and reach of your soldiers makes it both easier to round corners and makes them less vulnerable to weapon grabbing attacks from around the corner. [Yes, this is a real thing.]
  9. Night vision: even if there are lights in any place in the building, the first thing you'd want to do in any attack is blow them out and using night vision, effectively allowing you're troops to see and the enemy not.

The exact size of the hallways will dictate what exacly makes sense. If a light armoured vehicle can get through, then using infantry will almost not make sense for the main advances. The size would probably be that the vehicle can fit in a hallway lengthwise, thus meaning it can turn around in case nessecery and bring to bear it armamments when coming into a corridor.


Snakebots and Engineers

Buildings are masses of pipes, air ducts, and crawlspaces. Having some sort of drone(snake-like, multi-legged wall-crawlers, whatever) which could easily maneuver in those spaces would bring a huge advantage to whoever could figure it out. Equip them with gas/smoke grenades and suddenly your enemy must be on the alert 24/7. Give them some combat capability and suddenly every hole in the wall and air vent is a place where the enemy has to worry they could be shot at from. Not only that, but as battle progressed and the walls/ceilings/floors collapsed, such drones could still maneuver effectively. Your 10ft tall power-armored super-soldiers will have a hard time getting around once the battle has gone on a bit and they're buried under 20ft of rubble!

As a solution to rubble they'd probably really go crazy with combat engineer types, whose job it would be to maintain the battlesphere. Sure Right This Moment you need to blow that wall to flank that enemy to take that key position. But the wall is load-bearing and if it comes down your whole platoon gets buried under 30 tons of debris. Solution? Have the Engineers figure it out! I'm not an engineer by trade, but I feel like a lot of the equipment used to strengthen mining tunnels in loose soil would find its way into your force. As would more conventional building-support material. Otherwise the battle only lasts until the roof falls in. All your troops (at least the officers and noncoms) would likely need to get a TON of training in basic structural engineering so they would easily be able to identify key pieces of architecture. Both so they know what to bring down to bury an enemy, and what to leave in place/support/fix to ensure the same doesn't happen to them!


For the real world, read the American field manual ATP 3-21.51.

For science fiction, consider some gadgetry:

  • Drones (ground or air) with lidar and high-end inertial navigation gear for mapping.
  • Small unmanned ground vehicles with extendable ladders/platforms to reach the walls and ceilings in somewhat larger rooms.
  • Repair materials to reinforce structures and block tunnels you cannot defend. Some strong, quick-setting foam?
  • In your unit organization, a pool of "extra" squads to secure crossings without weakening the main maneuver units.

Indirect-fire weapons will not be used, since the ceiling gets in the way; this means everything from companies not having mortars to divisional-level forces not having things like rocket artillery or 155-mm pieces.

Those giants are probably going to be armed with rocket launchers and 37mm to 50mm artillery pieces.


Given the presence of power armour and giants, one type of specialist weapon might be rifled shot guns.

Against the power armour High Explosive Armour Piercing (HEAP) or High Explosive Squash Head (HESH) rounds would be effective.

Sabot slugs would have greater wounding potential against large targets than typical infantry rifle calibres.

In both cases I would expect the effective range to be somewhat less than an M4, probably around 300 meters for the slugs based on the performance of similar rounds designed for hunting. I'd have to speculate on the effective range of the explosive projectiles but 150 to 200 meters seems reasonable. However, in this environment the reduced range would be less of an issue, and I wouldn't envisage every soldier being equipped with one of these, perhaps around 1 in 4, so others would carry weapons capable of longer engagements when required.

While you wouldn't normally fire buck shot through a rifled shotgun due to the pattern opening very rapidly, perhaps in room sized engagements with a limited risk of collateral damage this would not be such a disadvantage.


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