Prices and placement of the defence target
The upgrades are way too expensive, so you will often find yourself or the defense target suddenly overrun, without any means to blast your way through it...
The placement of the defence target is also a small problem. It is placed so that if you shoot under you own gun (south) the camera doesn't show if something is attacking the defense target.
All in all it would have been a good game, if the prices would be lowered, but at the moment it really says "Go play something else"...