I find that upgrade is kind of odd.
Usually it just costs however much the turret cost, maybe a bit more, and it doubles the stats.
Also, just spamming shotguns or machine guns by the start will win you the game easily.
I would suggest balancing out the turrets. A couple basic ones with different strategies behind them, and a few medium ones with the same ideal. I like the buffer turrets, but I would suggest having them be more expensive and buff more.
As for the maps, a straight line/clear path would be better to let people plan ahead.
Yeah... if two attacks have the same speed, it's kind of confusing to wonder why one character's dead and it's still attacking.
Also, immediate hp/energy number drops will sort of help, instead of waiting until all the attacks are over to wonder whether you are at 1 or dead.