I really like this game, the simplistic artstyle works well, all of the enemies are incredibly interesting and stand out from each other with unique characteristics, and the combat, all though simple and limiting, works well enough to be passable. Sound effects are also alright.
First thing I want to say is that I felt the healing system was punishing. The amount of effort it takes to fill up the entire bar does not feel properly rewarded in the amount of health you get from it. The difficulty of the game isn't completely impossible, I died quite a few times but eventually made to the end, where I had a pretty thrilling boss fight with little health left. So I guess the higher difficulty was rewarding as a whole in the end, but in the early and middle of the game it can be a little frustrating and stagnating when you can barely recover any health when you get hit.
While the enemies are all fantastic, I feel like they could have been spaced out and more slowly introduced instead of throwing them all out at once. The problem with throwing the entire cast of enemies at the player in the first 2 or so rooms is that the novelty of them quickly where's off. You waste the opportunity to surprise the player later on with a new enemy, and keep them hooked by rewarding them by pushing further in. I know that it's a roguelike so it's harder to implement something like that, but there are probably still ways to delay certain rooms from popping up until later.
One thing I noticed is that whenever you died pressing esc just pushed me out of full screen and didn't actually return me to the main menu, so I instead had to just reload the page every time I died.
There was one room where I had to restart because I saw some of the eye tentacle enemies stuck in the wall. In that same room there was a jump that was impossible to make, basically you start at the lowest level and then need to jump further up to defeat the other enemies, but the two platforms going up to the second level had a floor on top of them, so when you jumped you would bump your head and be shot back down to the first level. Other than that level I didn't have any other level based technical problems.
I really respect what you've tried to do here, and I think it's a great shot to start with. Fantastic job with it!