The Crystal Gate

The Crystal Gate is a place on Astro-Knights Island. This is where you will find the portal to Mordred's Fortress.

Role in Astro-Knights Island
The Crystal Gate is located inside the asteroid field on Astro Knights Island. It's approximate coordinates are X-11 Y-81. Land there, leave your rocket, and climb your way up to the crystal gate. 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.

Trivia

 * Even after the key is pulled out and disappears inside the vortex, you can see it in the stone from in space.
 * After you complete Astro Knights, you can no longer enter the portal. As explained by the Princess, it's because the Orb is the only way back into the fortress.