The visuals are fun! The game progression felt empty-ish to me - I didn't feel I was having progress in the game. Mostly I could just avoid the enemies. It felt polished, the only thing I noticed was the restarting music, but making a loop is not trivial. Small peeve - a spaceship shouldn't be able to "move" back and pick up an item while flying forward!!11 :D And the flight felt "slow" to me, but honestly I have no idea how to make it better without making the game needlessly hard when navigating tight spaces.
Btw, an impressive engineering feat! Extra thanks for sharing the source, it was great checking out a modern game written in C.