Well done! A simple concept but executed well. Getting to choose between rooms was a nice touch, too - definitely adds some replayability. Very satisfying to mop up all the gold (nanoscales?) after clearing a room. I liked the visuals, too.
Ran into a couple bugs: I skipped buying the large gun, but it was added to my arsenal when I bought the huge one. Also, after I died, quit, and restarted, I ended up in a room with 95 enemies and all of my upgrades from the previous run.