There’s a real opportunity for explosion mayhem to ensue here as critters are blatted all over the screen.

Great arcade fun! The slowdown effect while choosing an upgrade always looked cool, and it was always clear how much health I had and how many enemies to beat until I could rank up. The only thing I would add would be some way to tell enemy health, they did not react much to the player's shots. Maybe changing their color based on health or adding health meters above the enemies so the player can see their health?

Fun! The gameplay is really satisfying on its own but the additional levels give you a nice challenge. The music and SFX are nice too. Would recommend adding some particle effects for feedback on the enemies as it starts to feel unresponsive after a while