Why are there no traps in this game?
I´d die to see some biter or spitter getting shredded in a trap which is set up a few chunks away from my wall...
What these traps would actually do is up to your imagination...a trapdoor can lead to many places^^
DK(2) has a lot of them (and I can think of many more, like a sticky strip of tar made from oil, which a) slows down and b) can be ignited for crispy fun - basically a real fire wall), see for yourself:
http://dungeonkeeper.wikia.com/wiki/Traps
After all, I got huge amounts of excess power and if shallow water would be pass-able combined with a motion detector "trap"...mwahahaha...*zaaaappp*

Or you could go to lenghts and set up a pollution center which draws the aliens there and then have a go with said traps...
most of these should be late game techs, except something primitive like a spiked trap and they should require some form of maintenance to not make it to easy and maybe have somewhat long recharge times