This was a lot of fun! I love tower defense games and this was a fun twist.
By the end, I felt a little like my puny ship wasn't really contributing compared to my army of towers. I wonder how the player can remain more relevant to the action. Maybe you can go to towers to upgrade them, or help defend them against an enemy that's attacking? Maybe the player's gun just needs to get stronger over time? Maybe some enemies drop huge money piles that disappear after a short amount of time, so you have to zoom quickly over to the good drops? I think there's lots of potential.