It need a better A.I.
I gotta agree with the guy who said you should give your other own bunkers a sort of AI control. They are pretty useless to have since your opponents can fire simultaneously while you have to keep changing bunkers to get them to fire.
My tip is, just make them auto-fire at a designed location (point your mouse there and select auto-fire, for example), as since you can at least choose what guns they will be packing, it will be rather easy to setup a survival strategy while you use one of the towers to take out the enemies.
Besides that it's a pretty good game. Just iron out those open bits and more people will come back to play it.