The crystal gate, and its Golden Key, are on the giant asteroid at X-11 Y-82 in the asteroid belt. You should only go there when you have defeated the three planets (jungle, ice, and fire) and have all three of the space knights with you. Drawing out the Golden Key opens the portal to the other galaxy, where the Princess is being held.
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.
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)
you fly to a large asteroid in the asteroid belt and land there. that is the crystal gate when you climb to the top
The Crystal Gate is an asteroidat the coordinates 11,18Full walkthrough athttp://poptropicasecrets.com/walkthrough/poptropica-astro-knights-walkthrough/
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)
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)
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.
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)
Go back down and pull it. You need the gate open to reach the cell. (see the related question)
You can't. The switch in the castle entrance only opens the small back gate. Later you can enter that way. (see 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)