Several things that I personally find could be improved:
-No need to rebuy extra slots for weapons. The last ship costs a buckload for the full package, making it not useful until you finished the final game (at least, I unlocked it after)
-Money needs to be smaller! Sometimes I lose track of bullets because the entire screen is flashing with coins, diamonds and bills. Perhaps making other attack sprites for the enemy could work as well.
I enjoyed this game, but it could be a bit cleared how much hp an enemy has, so I can calculate how strong my weapons have to be.
@Ocnic well that's weird. Be sure to check out their weaknesses so you have the proper guys on the field (can be done in advance when taking the mission. It's above the heroes you can select). Next to that don't waste all your money early on. I usually put 1 (but mostly) 2 guys out which can handle a couple of waves. Then I build a 100 gold dude, preferably the electric one with the HUGE range. Upgrade with 40% extra dmg @ one or two of your earlier dudes if you can't handle it . Also be sure to spam the correct skills in the heroes section once you can. With 35 magic use my mages pound through everything easily.