TerraScape is a turret defense game of the most straight-forward kind. You build turrets to keep the enemies from advancing.
You have a number of turret types at your disposal, with which you have to stop numerous enemies of different kinds. Some have protective shields, some heal themselves and some are partly invisible.
Download TerraScape [2.8MB] via Dropbox
I handed the following version in at the GMW contest: