The Crystal Gate is located inside the asteroid field on Astro-Knights Island. Its approximate coordinates are (11, 81). It is recommended that you do not land there until you have collected all of the Astro-Knights (Sir Cador, Sir Gawain, and Sir Pelasus). Once all of the Astro-Knights that you have rescued are gathered with you, it will be revealed that you are the Chosen One to pull the sword key out of the stone. A portal to another galaxy will open up and you will head to Mordred's castle in another galaxy.
The words on the Crystal Gate
Even after the key is pulled out and disappears inside the vortex, you can still see it in the stone from space.
After you complete Astro-Knights, you can no longer enter the portal. As explained by Elyana, it's because the Green Fuel Orb is the only way back into the fortress. That as well as the magic key, which you had used.
The Crystal Gate resembles the Fortress of Solitude from the Superman franchise.
It is possible to access the Crystal Gate before gathering all of the knights, however the portal cannot be opened until all of them are found.