So - back to the halfling rebel base:
The tunnels are well defended, so let’s talk about the rest of the complex. First off, the other rooms are not so completely cramped. There would be a couple of rooms where the ceilings might go as high as 5-6’, but offered the chance, the defenders would not fight in those rooms. What are the rooms for? Well, there are definitely barracks rooms. The rebels have some non-combatants down here with them, so women and children are housed. There will need to be some manner of kitchen, which requires that there be a smoke outlet. The thought was that the chimney would go up a tree, letting the smoke out amongst the branches/canopy, so it would be dispersed by the time it floated up, making the smell likely apparent, but not the tell-tale sign of smoke.
Most importantly, there are two laboratories. We said that everything in the complex could be explained. Now, we assume that the rebels could get weapons (spears, daggers, crossbows) but the labs build their traps and brew their poisons. Think about the advantages that the big adventurers have over the little halflings - They are faster - We took that away with the low ceilings. They do more damage - We took that away with the narrow halls. They are probably better fighters - We took that away with the narrow halls and the wider guard posts. The halflings are better with agility based weapons, thus we gave them the ability to shoot crossbows. Major spells will likely be slowed down by the twisting and turning tunnels - and explosions are as likely to kick backwards as they are to move forward.
So what do the halflings need to really even the score? They need to do more damage. How do they do that? Well, almost every weapon they’re packing, including their traps, is poisoned. It doesn’t have to be the killer kind of poison, just something that can bump up the damage when they hit, making every movement forward that much more dangerous for the invaders. The point is to be a nagging increase, not an all out death “spell”, but to be honest, either will work.
Meanwhile, the halls, the doors, the chests, the walls, everything is lined with traps. The traps can be simple: toothed jaw traps (bear traps), foot traps with punji stakes, trip wires to crossbows, nothing hugely clever, but add some poison, and all of a sudden, it’s a dangerous encounter.
That’s about the end of where I had gotten. The one issue I haven’t decided on is magic. How much, if any, would I give the halflings? Would they have some moderately powerful healers? Illusionists? In this crazy tunnel scheme, an illusionist can cause a lot more harm than a sorcerer. Alchemists? Don’t want to give the invaders anything they can use against the rebels, so that might not be right. I like the idea of some sort of “golem” that the halflings don’t care about. These weren’t supposed to be evil guys who were willing to kill each other. More like fanatics willing to die for their cause. Then, you could send a golem, skeleton, wind up robot, something out to fight in melee, without worrying too much if it caught a poisoned bolt in the back. With all the “evening of the scales” we’ve tried to do, the halflings would still be horrid in melee, so this could give them a better chance of fleeing.
What do we hope you’ll take away from this? Well, that if you think about it a little, moderately powerful defenders should be able to take on powerful invaders pretty easily. You just need to give them a chance to play to their strengths while taking away the strengths of the other guys. You don’t have to be out to kill every player character (and I was really out to do that with this dungeon). Just remember - the hardest fought battles are the ones the players remember the most!