I like the sketchy art and tower defense mechanic, great use of the limitation :) one thing I would comment on is might be good to be able to click on the tower sprites themselves when upgrading. Good game!
We agree, we thought of having a system where the player is sent back to the wave map directly after selecting the upgrade so they could e.g. drag and drop the upgrade on the tower they want. We just didn't have the time in the end !
