Nice Idea.. but could need a bit improvements..
Its good that you get bonus if u finish first, or that different creeps will spawn..
butcould need some more upgrades, more steps between it, or more research options..
just ad a bit more details, and maybe balancing it a bit, its way to easy, creeps should have more HP in later levels, and you jsut sit there and watch and wait, till u have the money for upgrades.. should be a bit more stuff to do while playing..
else nice idea of a circle TD 3,5/5