This game was way too fucking hard. I don't mind that enemies can attack your towers (and they can hit HARD), but there are so many factors that make the game more frustrating than fun:

1. Tower placement is very difficult. you usually can't put more than 4 towers in a group because of the retarded terrain. This drastically reduces the usefulness of repair towers and damage boost towers. Plus, enemies tend to attack the repair towers instead of the offensive towers. The enemies don't seem to be intelligent plotwise, so why do they attack repair towers instead of the machine gun shooting them in the face?
2. Too many aerial spawn points. The ground ones are generally ok, because you can funnel them in a certain direction, but for the aerial units there ain't shit you can do about their patterns, and having to devote towerspace to combat multiple anti-air paths in critical areas when space is so limited is such a pain. Plus if you don't have enough anti-air, the ones that can deal damage wreck the shit out of your towers.
3. Limited total tower amounts. WHY?!? It's hard enough already!!!

ran24 makes VERY good points...I got 100% trophies, but stopped playing the DLC after a few levels because of many of the points ran24 mentioned.

I'll be eyeballing the PSP release no doubt, I like the game a lot...