That's one of the best (if not the best) tower defense games. I've posted some (all?) of these before.

Action:
http://www.onemorelevel.com/game/boxhead_zombie_wars
http://www.onemorelevel.com/game/electric_man_2

Puzzle:
http://www.onemorelevel.com/game/bloxorz
http://www.onemorelevel.com/game/shift