Mostly fun game but with flaws.
It's fun shooting things. But, I'd enjoy it a lot more if it was designed with a more varied tower system. As it is, the optimum strategy is to simply upgrade a continual tower which is surrounded by a full arrangement of colours.
You've chosen a minimalistic style of tower and enemy. As such, more variety (cool down towers, swarm enemies, boss enemies etc) which require different tactics would improve it a lot. As it is the gameplay is simple to master, and gets boring fast.
Plus, the speed key is bugged. The towers are notably less effective at higher speeds. I'm not sure why. Perhaps their speed doesn't scale up as much as the game speeds increase.