Open this secret door, you need to step on a poison puddle to do so. Move through another poison puddle and reinforce with the two health chalices ahead.

You may now venture into this corridor. Beware the purple monsters in the niches. Break the mirror and neutralize the two ghosts. You will notice the captured girl, and more purple monsters, in the room behind the hanging skeleton.

IMPORTANT! DON'T open this secret door before you have both the brass key AND the iron key. Otherwise you will get stuck!

Shoot these two guards. Reinforce if you need to. The silver key is behind the two barrels. Blow them up. The moment you do Zombie Hitler will come at you from the cell he is in. Don't let him surprise you! Get the silver key in time (even if you have to rush through the poison puddle), then run westward steering clear of the death chalice in the middle of the hallway.

Tour the floor all the way here, but be on the look-out for Zombie Hitler. With the silver key you get access to this room and the bronze key. With the bronze key you can tour the whole floor both ways making it easier to avoid being cornered by Zombie Hitler. You will also get the 1-up in the cell he was in.

Continue.