Very interesting game, with a few flaws.
- I don't like the ramping up in difficulty,, because you have no idea what to do when they are coming out from every spawner. Maybe sticking to one spawners or two would have been a better idea. Or making several levels with increasing difficulty.
- The towers have no attack animation which doesn't feel good. I know that you had limited time, so this is not taken into account here.
- Missing music and sound effects are also not that great. Those play an important role. Just a little shot sound for the towers would make it a lot better. But same as last point, not taken into account.
- You are also missing an indication how many enemies are still left. i.e. when you finished the level, because I don't know if there is an actual finish.
- One small thing, the enemies don't spawn when you play a second time.
But overall nice idea. Just had to take away some points from the design category.