This was really fun and addicting. Here's just a few constructive criticisms.
A few bugs I noticed:
If you put a building on yourself or an enemy, it has a chance to launch yourself up or push yourself through the floor.
After restarting, (or dying), the cost of your building, and their sizes does not change. This can lead to getting very powerful attacks at the beginning of the game. ( This might not be a bug)
Other criticisms:
Another couple of buildings would be a great addition to the game. I especially wanted a speed building while playing.
While the waves signified stopping points in the game, they felt distracting when they appeared with a bunch of enemies around you. Maybe causing the wave to end when all the enemies are gone would improve this.
All said, this is a great game and was a lot of fun. Continue the good work!