you can do somting
The key in the dungeon doesn't work. You throw the switch and come into the cell from the back way. (see the related question)
Throw the switch when you get the cheese. You can enter the dungeon from Mordred's underground laboratory once you have the robot owl. (see the related question)
The only things you do in the castle dungeon (under the library) are pick up the cheese and throw the switch. (see related question)
To reach the lever on its back, jump up on the chains and cross to the back of the dragon. Avoid the spiky tail and throw the lever when it is to the back.
If you can't pick up the cheese or throw the switch in the dungeon, you could try going out and coming back in. Otherwise, reboot Poptropica in your browser. You can't complete the island without doing this.
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)
When you get the moldy cheese from the castle dungeon, throw the wall switch that opens the rear gate to the cell. Later, you can enter through the tunnel from Mordred's lab.
You can't open the cell from the library side. Get the cheese and throw the switch that opens the back gate to the cell. You can enter the back way from Mordred's underground laboratory. (see related question)
You can't open the front of the cell. You have to go in the back way. Throw the switch in the dungeon and then go back out. You can enter from the tunnel to Mordred's secret underground lab. (see the related question)
You really can't do anything else once you get the cheese and throw the switch. You will have to return to the dungeon another way, one that accesses the rear gate that you have opened. (see the related question)
Although you find the captured robot invader in the dungeon, you cannot get to him from the library. Throw the switch to open the back gate to the cell, and return through Mordred's secret laboratory tunnel. (see the related question)
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.