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).
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.
One of the robot attackers was captured and put in the castle dungeon.
The dungeon is in the castle library.
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.
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).
There is no jail on Astro Knights Island. There is a castle dungeon, where an attacking alien robot is being held after his capture. (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.)
Go to the library and then there will be a sign that says McM. Click on it and you're in!
in the castle library click mcm in the non-fiction section to get into the dungeon
The dungeon on Astro-Knights Island in Poptropica is located inside the castle on the left side of the main room. You can access the dungeon by climbing down the ladder next to the statue of Mordred on the top level of the castle.
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.
There is a hidden stairway to the dungeon in the castle library, and a hidden opening under the fountain.