Pretty good game. Only two major complaints.
First, the difficulty ramps too quickly, with level 1 being basically impossible to lose and level 2 being basically impossible to beat without serious exp grinding.
Second, it's awkward to switch between building and shooting. You have to stop shooting to click build, hotkey the building, click the building location, click cancel after construction starts, then click weapons, hotkey your weapon, and then start shooting again. Ideally, all weapons and buildings should be in the same tab, and you should be able to click/hotkey any of them at any time. The construction cursor should also disappear after you place one, so that you can immediately begin shooting again. Maybe hold shift for the multi-place?
Other than those two things, the game is pretty fun, and I could see myself playing it all the way through.