Numion said:
Maybe we would need a bit more information about the situation. Those traps are very expensive, and you'd need very many traps to wear out a cautious group.
Don't use the expensive traps. Matter of fact, don't think of the traps as the weapon; they're herding systems. They slow the PCs down and put them into places of the Kobold's choosing. Someone brought up the Viet Cong and that is really the attitude you want to have. Feral cunning with a finite amount of technical/mystical resources but lots of raw muscle power. Imagine how much work 400 kobolds who've never heard of OSHA or the 40-hour work week can accomplish.
First thing is that all tunnels will be kobold-sized and most of the time only big enough for one or two kobolds to be side by side. All medium creatures are immediately disadvantaged; Movement speeds are reduced from crawling and in some tight-for-a-kobold-places reflex saves may not be possible. Longbows and all reach weapons should be useless. It also reduces the amount of firepower the PCs can bring to bear since only two or maybe three PCs can act on the kobolds. Killing kobolds will even make life difficult because it reduces the amount of maneuvering space by that much.
Second, lots of turns. There will be no long lines of sight. It should be as twisty as koboldly possible and be designed for 3-dimensional confusion, meaning small ladders, drops, tunnels appearing at different elevations, etc. PCs should have great trouble navigating.
Junctions will be the only open area and will be no bigger than absolutely necessary, at most 10x10, and will also act as quarters/storage for the kobolds. Junctions should always have at least 4 doors, that way the kobolds can outnumber the invaders. With 3 doors out, 1 door in and the tight confines, the Kobolds should outnumber the PCs by 3:1 at each turn. Whatever items happen to be in the junction (crude crates, sacks, filthy straw on the floor) will be cover to the kobolds. PCs should also constantly fear being flanked as it will be almost impossible to completely clear out the warren.
The interconnected tunnels are also handy at controlling spells. Bursts and rays will be absorbed by the walls; emanations & the like could easily work their way back to the party.
Traps will be simplistic. Punji sticks, concealed caltrops, murder holes, boiling oil, false floors, scorpions, snakes, hungry rats, and wasps nests. They won't even be "automatic" traps as much as remote triggered by a lookout. Lots of spiked wood panels that drop from the ceiling, deadfalls so hallways collapse, etc.
And the traps won't be to do damage, not really. The trap will keep the party where the kobolds want them for a bit. IIRC, dire badgers can burrow so when the party feels safe the badgers will burst in from the sides and begin sucking blood. Maybe the kobolds will build smokey fires at both ends of the tunnel the PCs are trapped in. Or just lead them to the very bottom of the warren and then break open a wall to a well so that a thousand gallons of water (or maybe sewage) drowns them.
The best part of kobolds is that they operate with little chain of command. They don't know what radio is and they vary rarely use runners to carry out orders. Their plans are simple (Shoot'n scoot) but effective and you can't take out the army's brain because they don't have one they can't live without for a while. Heck, most of them wouldn't even know about it if you did so what's the point?
This should turn into a bloody, painful exercise where the party is nickel & dimed to death. Not that they'll likely die, but they won't exactly have a "win".
I recommend checking out a book on Vietnam from the library, particularly one addressing the tunnel rats. Replace "AK-47" with "Lt. Crossbow" and "VC" with "Kobold" and you'll do fine.