Get the yellow key. There are two yellow doors, one from the upper, one from the lower room. Pick one, get to the oval wooden room. Inside the room, a skull switch. When that is pressed, a secret room opens across from the lower yellow door.

Opposite the skull switch, tap the wall. A secret room opens, containing stuff that wil help you to deal with your... guest.

You can find these, but unless you cheat, you won't get credit for them.

Tap the left wall inside the secret room from #2; another secret door opens, with a goodie inside. Secret #3 is the door itself, but you can't get it; the door is too narrow - your center has to be over the threshold to trigger it. Secret #4 is the shelf with the goodie. And you can't get this one, because the shelf is too high up. Bummer.