I am having a very hard time. I've destroyed the bandit base and I made…

I am having a very hard time. I've destroyed the bandit base and I made the well got the note, and found the key. I've read the spoiler in the game guide but I don't find it specific enough to know exactly where to find the door that it unlocks, is it in this same level or a different level? Can someone please tell me exactly where to find it and in which level?