Amazing game =) A few balancing things
1. In the Northernmost shop, you can gun there, and all of the enemies can funnel into the area. This makes any other area rarely used, as you're around the store, and you can mostly live perfectly. You can fix this by embedding the store into the edges of the map, so there is no way to do this, or put them in the center.
2. The start feels a bit too easy, as the start screen teaches you the mechanics, the other parts of the game feel boring, until you get to where a lot of enemies spawn.
But overall, AMAZING game. This deserves a TON of praise, so keep it up!