I liked it, but 3:00 feels kinda long for one wave. I think maybe 2 minutes with slightly more enemy spawns would be nice.
It'd be nice to show attack range when you place buildings.
Bug-wise, I think the tower is invincible after you retry a level, and I can't seem to build 3x3 size buildings flush with the back sides of the tower.
I think the color scheme works well with the low complexity, it feels like Google Snake to me.