In the final level, I try to lead all enemies to a specific point using barricades - that works very well, except for Kobold Sappers. These are destroying my barricades (with their backpacking bomb). What is a good strategy to prevent them doing this? Beside killing them as early as possible, obviously.
Well a few suggestions as the kobold sappers tend to be the first ones out of the doors and all, faced the same problem as you before.
1) brimstones, since the kobold sappers just burst with a bit of dmg from anything, this seems good when a level resets to the next wave. but have to place them further away from the barricades due to the explosive radius
2) tar pits and archers seems to work as well but under the constraint of gold and all it might not seem to be a good option.
3) tar pits and arrow traps, as the tar pits will slow the kobold sappers and make sure they cluster together before reaching the arrow traps, killing one in the proximity of another will cause a chain explosion.
4) concentrate some of the suggestions on one side of the final stage and use the knowledge weaver or the one that grant you increased movement speed for you to run and flank the sappers by yourself, since one hit will grant them their death.