There is no way into the cell from the library. Throwing the switch opens the "back gate" to the cell, which you can access from Mordred's underground laboratory.
(see related question)
The small gate is in the BACK of the dungeon cell, and connects to the tunnel Mordred dug. By throwing the dungeon switch, you open the gate so you can come in that way. (see related question : getting back in the dungeon)
Push through the dirt wall. You should already have opened the back gate with the switch in the dungeon. Feed the mouse to Merlin. He can retrieve things for you.
You have to come in the tunnel that Mordred first dug to escape. When you flipped the switch in the dungeon, you opened the gate at the back of the cell. You can get in that way if you reach Mordred's hidden laboratory.
Go back down and pull it. You need the gate open to reach the cell. (see the related question)
The switch in the dungeon opens the back gate to the cell. Later, you can enter that way through Mordred's underground laboratory. (see related question)
The coordinates are X 11, Y 82
Back Gate to the Dungeon CellThe front entrance to the dungeon cell does not open. Throwing the switch opens the rear gate, which is accessed from Mordred's secret laboratory.The Golden Key and the Portal GateYou need all three knights with you to pull the Golden Key from the Crystal. This means beating all three planets.(see related questions)
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)
The gate to the dungeon cell is controlled by a switch at the entrance from the castle library. When you enter the dungeon to get the cheese, you should throw the switch. Later, when you access the dirt tunnel from Mordred's lab, the gate is open to the cell. You need to feed the cheese to the mouse and the mouse to the owl to recover the fuel rod from the dungeon. (see related question)
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.