After you enter the castle dungeon through the secret entrance in the library, all you do is throw the switch and grab the cheese. After that, the librarian and the guards won'y let you go back down.
So to go back in, you get the key to the underground laboratory (beneath the hay bales by the mill). Once you get into it, there is only a dirt wall to stop you from reaching the back entrance to the dungeon cell. Push hard and the dirt collapses.
(To get the mouse, key, and owl, see the related question.)
keep pushing it
You suck you spelled knights wrong. Hahaha!
walk through
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.
The library slip is needed to get into the Dungeon through the castle library. It provides the location of the secret passageway. Get the gold coin from the fountain and pay your way into the Museum. The library slip is in a stack of books next to the museum operator.
The cell is only "blocked" by the bars on the dungeon side. When you throw the switch, it opens the back gate, which leads to Mordred's underground lab. (see related question)
you push through
In the dungeon under the castle library, pick up the cheese and pull the switch that opens the back gate of the cell. In Mordred's underground lab, once you feed the mouse to the owl, you can go down the passageway to the cell gate. Next to the bars, the wall is dirt and you can just push it down.
The island that uses spacecraft to travel through the quest is Astro Knights Island.
After the Astro Knights island release in June 2009, the next regular feature was Poptropica's Haunted House in October 2009. (Available through the Poptropica Store) The next scheduled island was Counterfeit Island.
Walk out the passageway to the left, which leads to the falling block puzzle and the entrance to the Sphinx.
In the lower aisle of the castle library, in the Non-Fiction section. You cannot use the secret passageway until you have the Library Slip from books at the Mordred Museum.