Once you have the Library Slip from the Mordred Museum, you can enter the dungeon from the castle library. Click the shelf marked "McM" on the lower aisle and enter the dungeon. All you can do is to pick up the cheese and throw the switch on the wall, which opens the back gate to the cell with the captured robot invader.
you can do somting
To see the solar system on Astro Knights island, go to the Planetarium. (not part of the game, but an interesting view)
next to a bookshelf.
go to the secret door
IS IN DA LIBRARY... SOMWEAR.... and i dongt
In the palace the right door
You don't need to.
There are two books in the library and one in Mordred's laboratory.
You do not get or use a bucket on Astro Knights Island.
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 is no treasure chest on Astro Knights Island.
under the library, you click on the McM label and stairs appear