Sure, the keys are only in non boss levels, excluding the level with the pentagram. The first one is found after opening the metal door that says locked in the first level, the lever for it is behind the pillar in front of it. The second one is a bit tricky but after you pull the levers for the doors, at the one labeled C across from it is a hidden door which has the key in it. In the third level it's behind a hidden wall next to one of the four pillars in the corners of the room, the fourth is in the room where you have to pull a bunch of levers to open up a gate to the teleporter, after you pull the last one to open the gate to the teleporter, it should spawn where the lever is to the right of the teleporter. The last one is at the end of the level where you choose the boss, you have to find the lever labeled C and once you open the gate it should be right behind it. Hope this was useful.