itch.io

There are areas with "infinite hordes" (when you enter the area you get that text that comes into view with red border announcing infinite hordes). Basically you kill one and another pops up. They are meant to be challenging to get through, but usually have a perk you can unlock.

EDIT: I mention this because I literally stayed in an area for 30 minutes fighting my way through before I realized what was going on, and I was almost done with the game before I noticed the notification whenever I entered those areas -__-