Very nice game, simple but enjoyable, especially for your first try it absolute decent!!!
Just a few things to think about
1. As for me economic pace kinda slow, after 1st wave, as a player, i already want to buy something
2. When your both arms on keyboard (cause of control), you dont want switch it to mouse to start new wave, maybe set it on space button or anything else
But its only my opinion, maybe other people see it different, anyway as i say its very nice touch, keep making games!!!