Nice man haha. It reminds me a bit of an old tower defense game I played, I can't remember the name... but in that game, you'd basically make the path the enemies have to walk using the towers which would block them off. I actually left the game on once I got to level three and let it keep running, it was funny to see how many enemies ended up stacking up (I may have triggered a glitch where I was invincible, surely right? haha). Anyways I think it was pretty cool. I think the 3 minute timer for levels was fine for this project, but if you wanted to deepen this game maybe make the timers a bit shorter and add some more waves.
Oh also, my favorite part maybe was in level 2 when the yellow dudes started coming, and just leaped over all of my walls ahaha. That was a good surprise.