World Invasion TD

I made this game in order to learn how to make an upgrade system. I also had a lot of plans for it but hit a roadblock at some point so I put it on hold. I may restart developing it again seeing as how I already have all the story written out and a couple of assets complete, but that is only after I finish my current project (a turn based RPG). It currently has 3 fully playable levels (with place-holder graphics) and an upgrade system. Currently it is pretty fun to play so by all means you can t...