Between the unique upgrades, multitude of enemies with not just enemy movesets, but unique abilities for the player, there is a crazy amount of content here. And it’s really polished on top of that! The artstyle is really neat (reminds me a lot of Runescape), and the upgrade descriptions were pretty fun to read.
The lack of any real fail-state and a defined end point does hurt the replay value, but I think the fact that the worst thing I can say about the game is that it ends, says a lot.