To open the staircase in the castle library, you need the Library Slip from the Mordred Museum. Use the gold coin from the fountain to pay your admission, and you will find the Slip in a stack of books. Once you have it, clicking the McM shelf in the lower aisle of the library will open a passage to the castle dungeon.
it is McM
you click on the paper under mcm
under the library, you click on the McM label and stairs appear
if you mean in astro knights, the mcm is in the library in the castle
McM is the shelf label for the lower aisle of the castle library. If you have the Library Slip, you can press this shelf to open the dungeon passageway.
in the castle library click mcm in the non-fiction section to get into the dungeon
Go to the library and then there will be a sign that says McM. Click on it and you're in!
McM
The library SLIP is marked "Non-fiction McM", which is the lower aisle in the library. Press the spot on the shelf marked McM.
The McM is on the bottom floor in the library. You can click on the mcm if you have the library card. The library card is in Mordred's house museum.
The rolled up paper is on the shelf in "Non-Fiction McM". This is written on the library slip found at the Mordred Museum. (Get the shiny gold coin from the fountain and pay your admission.)
The dungeon appears if you click McM, after you got the library slip from Mordred's House/Museum. You can get the coin from the fountain, where it is shining.