Nice short little game, liked the concept of different enemy types (which could definitely be expanded further, like special shield types, or de facto types like enemies which move too fast for certain turrets to snipe them), shame it ends so early though.
The balancing is pretty good for the first few rounds but after that the rate at which your turrets destroy enemies gets so high that you can't even see what you're fighting most of the time since the enemies just get obliterated at spawn.
The text at the top right explaining what to do could be changed so it only displays the instructions for one specific building.
I liked the simplistic, kind of nostalgic visual effects of the game. Not a real big fan of the audio though (especially there being no way to turn it off)