I really liked the FPS/Tower defense mashup. I didn't notice the enemies coming in the first wave at first so I took some damage but once I got the loop down I was able to push them back and win.
I never had to go back to old towers so having multiple of them wasn't too interesting. Might as well have been upgrading the first tower. I guess it does provide that sense of pushing the enemy back and gaining territory but it wasn't a bit player in the game. Not much time to explore the area so I never figured out what the big tower in the background.
I think with some polish and and a bit of a separation between the gathering sections and the tower sections you'd have a solid game here.
Thanks for making this!