Creative concept! It took me a little while to grasp though, it wasn't clear on my first few runs, resulting in game overs seemingly coming out of nowhere. Once I actually understood what was happening, it clicked and it caused many interesting gameplay considerations.
For example, when all coins gained enemy behaviour, I had to evade everything and make sure no coins spawned. Which is completely opposite of your instincts to blindly grab treasure chests and kill enemies. This made me constantly re-evaluate the circumstances and adjust my playstyle. Great use of the theme!
Presentation and controls were good, the shooting felt nice and punchy. The main player graphics were a bit lacking though, it didn't make much sense to me that a generic skeleton would shoot without having any ranged weapon.