Cool game.
I would suggest some sound effects. It will instantly transform this game to feel more fuller. Also add some death effects on the enemies. It will give a sense of accomplishment to the player. It would be better if the waves didn't appear over each other, maybe wait for the player to kill all the enemies before spawning the next wave. I could also walk from the path onto the wall tiles.
Another thing, the font is not really legible for long texts.
Overall, well done!