Doesn't start in firefox for some reason. But works good in chrome.
The UI could use some work. Gameplaywise you should add health pickups in the dungeon. I am a casual I need hearts to survive. Also don't throw the player out so unceremoniously. Give them one second to understand that they died.
Couldn't beat the first dungeon, so can't talk about the upgrades. Stealth mechanic works good tho.