Nice work! The game was pretty polished and fun to play. There's a decent amount of small things you could add to make it better though, that being:
* a way to recover health
* displaying the names of powerups
* changing the color of gray tanks to make them easier to spot
Having the walls shoot you is a good idea, though you'll usually only get killed by enemy tank fire. If you were able to choose weapons, and make the wall bullets more dangerous, you could have a great risk/reward system where choosing the more powerful weapons would be able to kill tanks easily, while posing the threat of the walls shooting back at you with great force.