I enjoyed this game very much. Cool mixture of tower defense and shooter. Also there's a lot of stuff to buy and you can keep leveling up and collecting money for some good reason. The graphics were good as well and the general gameplay was enjoyable. Nevertheless sometimes the player's mech gets stuck on invisible edges of towers or whatever, which is kind of annoying. Also, it would be nicer if there were some more different weapons and towers, but all in all it's really fun.
Btw, couldn't experience any unbeatable levels, to be honest. In my opinion most levels on regular difficulty were much too easy. But I guess that's where the hardcore difficulty comes in handy.