I had a really good time with this! I replayed it a few times after winning, it's honestly just really fun to fill the screen with units and watch them blast those nasty dudes.
Great use of the limitation, I love how literal it is and it meshes perfectly with the genre of your game.
Also, I love the face on the bus. I like to think that, at first, he's afraid of the baddies shooting him, but by the end he's horrified at the decimation I'm dealing to this neighborhood.