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)
The coordinates are X 11, Y 82
Once the Golden Key and the Green Orb are returned to Arturus, there is no way to open the crystal gate.
You can't. The switch in the castle entrance only opens the small back gate. Later you can enter that way. (see related question)
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)
To draw the sword from the crystal stone, you must have defeated all three planets (jungle, ice, and fire) and have all three knights with you. (see the related questions)
Feed the mouse to Merlin, the mechanical owl, and he will follow you into the cell. Click Merlin, then the vial, and he will retrieve the fuel rod for you.
you fly to a large asteroid in the asteroid belt and land there. that is the crystal gate when you climb to the top
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)
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.
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 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.