next to a bookshelf.
It is the lower aisle in the royal library (castle, first floor right), and the shelf McM will open the dungeon entrance if you have the Library Slip.
The moldy cheese is in the castle library entrance to the castle dungeon. Get the gold coin from the fountain, pay your admission to the museum, and get the Library Slip. Now when you go to the castle library, you can click the shelf (McM) that opens the stairway to the dungeon.
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In the castle library, on the Non-Fiction aisle, there is a scroll marked "McM" which opens the hidden dungeon entrance. You need the Library Slip to access this, which is in the Mordred Museum (use the gold coin in the fountain outside).
The dungeon is accessed through the castle library (first floor right). If you have the Library Slip from the Mordred Museum, you can open the hidden stairs by clicking the McM shelf on the lower aisle (non-fiction).
Once you have the Library slip, you can press the scroll McM on the lower aisle in the castle library, which will open the dungeon stairway.
under the library, you click on the McM label and stairs appear
you cant i have that same problem too
The dungeon entrance in the library does not use a key. Get the coin from the fountain, then the library slip from the Mordred Museum. Now you can open the secret passage in the castle library (Non-Fiction McM). However, once the dungeon is guarded, you will need the key to Mordred's laboratory to reach the cell again. (see related question)
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.
Pick up the moldy cheese, and flip the switch on the wall (it opens a back entrance into the cell). (NOTE : once you do these, you cannot get back into the dungeon through the library. If you cannot find another way in, see the related question.)
Use the cheese from the dungeon (under the library in the castle). (*To get to the dungeon, see the related question.)