I started making a tower defence clone at the end. For those of you who don't know it, you should check out the lounge thread about it.
Update: Showcased! Download the game or view screenshots.
I'm making the levels more balanced. The gameplay is not finalized yet so if you have suggestions, go ahead and throw them in.
You have choices when upgrading towers: You can upgrade one of the three stats Damage, Speed, or Range with each upgrade. Towers differ in their upgrade factor and number of times they can be upgraded.