I've made up an excel map (apologies to piratecat) of a sample kobold lair. This is a side on view, and only outline the basics of the place, I don't think that the rest would be relevant to the players anyway. Here is the key:
The entire lair is dark
White= passage that the players can travel through, they are not wide enough to use a large slashing weapon, with the exception of the large rooms.
Red= Passage that medium players cannot fit through, designed for kobold use.
Green= pit trap, spikes at bottom, and poisoned spikes up walls. The top is covered with wood strong enough to support a kobold, but not anything in heavy armour or of medium size.
1. This is the main entrance, there will be about six or so kobolds at the far end, armed with crossbows. Upon sighting the party, they will each fire once, before retreating behind the door. Four will remain, continuing to fire from cover, while the others will retreat up the ladder 4, and ready themselves, above the pit traps.
2. This is the observation deck. After seeing the party enter, one kobold will alert the rest of the lair via a sending, while the other five will move to the end of the trapped corridor, ready to shoot the party as they advance. Note that they will be out of sight of the party.
3. A pit trap if the barbarian charges.
4. This is too thin for medium size players to climb, but the kobolds can retreat up the ladder when necessary
5. This corridor is filled with pit traps the purpose of which is to buy the rest of the kobolds time to prepare themselves for the attack. While the party advances, they will be fired upon from the other end by the kobolds previously residing in 2. Each pit trap has a hole above through which the kobolds can shoot arrows, dump rocks, or do any other activity which you can think of. When the party gets near the end of the passage, the kobolds will retreat and others will fire from 4. This way. the party will always be under fire, and forced to act. More kobolds will arrive, and fire at the party from a distance once the rest of the base is aware of the group's presence. Keep the players under fire, from a large distance, to force them to move onto the next stage and/or use up precious resources.
6. This is a water filled passageway designed to seperate the party and extinguish any torchs they may have.
7. When the lead player emerges from the water he will be attacked with crossbows from this point
8. After the party has moved on, this point will be collapsed in order to trap the group.
8 (the other one). This large drop has a ladder running downwards into darkness. If the party climb down, they will be innundated with arrows from the kobolds accross the other side. The ladder is also trapped, and can be severed by a kobold at any time. This will most likely be done when only one player is on it, in order to seperate the group. Hopefully, this will ensure falling damage, or at least use up some spells. The fallen player can then be attacked from the other vantage point.
9. A similar water idea, with the lead player being attacked after emerging, this time though a larger climb is required, still under fire.
10. This is the kobold "barracks" as such, where most of them will be coming from.
11. An area where the players can be attacked from above, again.
12. When the players have reached this point, after another climb under fire, a fire will be lit from underneath, forcing the players to move into area 13, or suffocate.
13. This is the "passage of death", the kobolds will be fully prepared, and keep up a constant barrage of fire from the far end. The passages above will allow freshly boiled oil to be poured, and spells, arrows, etc to be fired. Kobolds will also be inside the pit traps, ready to attack the players as they pass above, or engage them after they fall.
14. This is the final room, where the toughest kobolds will reside, ready to take on the depleted PCs. Illusions should be used to make it seem that they are at the far end, whereas they are in the middle of the room. Swarming tactics can be used, as almost every remaining kobold will be here.
15. This is the "shooting gallery" that someone else spoke of. While the players move downwards, they will be fired on by dozens of kobolds.
I know that I've probably missed a few things, but, oh well.