Nice game, though a bit too hard for my taste.
I think that the main focus of the game should be rapidly connecting the right type of tower in the right sequence in the right place (for example making enemies wet with water and then attacking them with ice) rather than getting the money to add more tower as in usual tower defense games: I would make the tower way cheaper (especially the air tower, that more often than not took forever to aim at a target and either missed or never even fired) so that you place more of them and then focus on activating them in the most efficient way.
For the same reason I would remove the middle click and just have left click to select in the store, place a new tower or quick connect and right button to sell a placed tower.