This heavily relies on quite a few variables, how is your access to minerals? Do they have each access to copper and tin for bronze? Is iron easily accessible or unlike our own history is iron quite rare? Are all those metals rare in sufficient quantities and we rely more on bone and stone? The limited amount could be used to force weapons for an elite well funded force but the average soldier won't, etc.
Your smallest tunnels will be prime choking points, fortifications like gates will be build at them. Unlike the real world going around them is near impossible. Just put several layers of thick doors down there and you're good. There won't be space to bring in a battering ram, the walls are solid rock. That gate will hold against anything but a large army backed by a powerful nation.
Conclusion, you'll rely on siege warfare.
See there is one way around those gated choke points, pickaxes and general mining. Digging mines has been done since the stone age. Your cave people will be able to dig tunnels. Now the speed of that depends on the above mentioned availability of metals. But even without those mining will be done and tunnels will be dug.
If a sufficiently large and organized society arises I'm pretty sure they're dig their own highways. And their armies will dig their own entrances into enemy territory. This will also be almost impossible to track. You'll need an extensive network of listeners at your borders, maybe water alarms as well. You need to know where the enemy might breach your caves.
My point here is, fighting in the small tunnels won't be done by organized armies. They'd build their own tunnels large enough to comfortably fit their warriors. Depending on the accessibility of metal those men will likely evolve a hoplite like system of heavy spearmen. They can hold a tunnel for great lengths of time. And unlike on the surface world can't be flanked. This will be the core of any advanced nation, heavy spearmen for defense and offense.
Conclusion, heavy infantry with spears and shortswords.
For ranged combat there is a clear pressure to remain small, there's no room for a longbow or javelins. Though given the focus on spear warfare by the elite warriors some usage of throwing spears is expected. But the kings of ranged warfare in your underground kingdom will be slings and crossbows.
Slings went out of favor because they're out performed by a bow with comparative skill. A sling takes quite a bit of skill, but they're easy to make. Crossbows would be well established by 1100AD. They started to be more widely adopted in the west around the fall of the Roman empire. In China they've been part of regular warfare for thousands of years.
Conclusion, crossbows will play a limited but effective role for defenders.
Your military would be a combined force of spearmen with shortswords and archers with crossbows. Thinking on it given the short range, a repeating crossbow makes a lot of sense.
You're trading range for speed and size already, might as well replace range and power with a fast rate of fire. But much like real history their usage has a limited place on a pitched battlefield with professional soldiers because those guys wear armor.
However, raids, bandits and vanguard forces are an entirely different thing. They'll crawl the small tunnels for unknown routes. They'll be confined, struggle with sentries and the like.
Those men have no interest in shields, long spears and other cumbersome things like long rapiers. They'll go for short things, daggers, an armored gauntlet, maybe a bronze age rapier.
They go for short blades, probably augmented with a sling. They're easy to carry, plenty of ammunition around and are quite precise at short range.
In summary:
- Heavily armored spearmen with shortswords using a shieldwall like
formation.
- Assisted by crossbowmen.
- Repeater crossbows for militia
defense.
- Bandits will rely on shorter weapons as they'll shun the
larger tunnels.
