Good game for an occasional evening of diversion. I did encounter some bugs, though (besides the alien ones that spawn to kill you :D)
1) The aliens sometimes get stuck inside the spawners and then just sit there.
2) Probably part of (1), but path-finding is flawed, causing things to get stuck. For example, aliens will go after a turret in a mostly straight line and not go around obstacles in the way.
3) As a consequence of (2), players can exploit it by building unfinished turrets in front of finished turrets, and never complete them. Aliens will go after the finished turret, bump in to the unfinished ones, and then just sit/stand there while the finished turret(s) kill them.