under the library, you click on the McM label and stairs appear
Go to the library and then there will be a sign that says McM. Click on it and you're in!
Use the cheese from the dungeon (under the library in the castle). (*To get to the dungeon, see the related question.)
There is a hidden stairway to the dungeon in the castle library, and a hidden opening under the fountain.
The only things you do in the castle dungeon (under the library) are pick up the cheese and throw the switch. (see related question)
There are two entrances to the dungeon : one is in the castle library, the other in the secret laboratory under the hay bales (this leads into the rear of the dungeon cell). (To enter them, see the related questions below.)
The library is on the first floor inside the castle (right side). Before going there, find the gold coin in the fountain next to the museum. Use it to pay your admission and get the library slip from the stack of books. This will let you access the dungeon under the library.
The hidden stairway to the castle dungeon is under the library (first floor, right). Get the gold coin from the fountain and pay your admission to the Mordred Museum. In the stack of books is a Library Slip. Once you have it, you can press the Mc-M shelf in the lower aisle of the library, to access the hidden dungeon entrance.
There's a lock under the hay in Astro Knights
The cheese is in the dungeon under the castle. Go to the fountain in the square to get the gold coin. Use it to pay admission to the Mordred Museum. In the stack of books at right there is a Library Slip. Once you have that, go to the castle, enter the Library, and click the shelf marked Mc-M. This will open a stairway down to the dungeon.
It's downstairs. Press one of the stacked books with a paper on it, and it should get you the library card.
There is some moldy cheese in the secret dungeon under the castle.To get the cheese:Get the gold coin from the fountain.Go in the Mordred Museum and get the library slip.Go to the castle library, the non-fiction section (lower aisle), and push on the "McM" shelf.All you can do is get the cheese and throw the switch on the wall.