you go into the portal. then you go to the bottom and talk to the princess.
The three knights tell of a mysterious planetoid in the asteroid belt, where a portal to another galaxy exists. Travel to the asteroid belt and to X-11 Y-82, where you can pull the Golden Key and find the Princess.
The giant asteroid is at X-11 Y-84, but you can only remove the Golden Key when you have beaten all three planets and have all three space knights with you.
Mordred is somewhere in space. Defeat the three planets and you can find the Golden Key (on the giant asteroid at X-11 Y-80). (see related question)
you just click it.
The Golden Key is Mordred's gateway to another galaxy, hidden on a giant asteroid in the asteroid belt (X-11 Y-82). In order to pull the Golden Key, you must have defeated all three planets (Jungle, Fire, and Ice) and have the three knights with you.
The small key in Astro Knights Island is for Mordred's secret hideout. When you come to that wind mill area, move the hay bales apart and use the key on that.
Once you have beaten the three planets (jungle, ice, and fire) and have the three space knights with you, travel to the asteroid belt (lower left in the solar system). There you will find a giant asteroid with a castle, and the Golden Key which opens the portal to another galaxy. The secret planetoid is at X-11 Y-84. You MUST have all three knights with you, or you cannot use the Golden Key.
Once the Golden Key and the Green Orb are returned to Arturus, there is no way to open the crystal gate.
the door that's locked
The giant asteroid with the Golden Key is near X-11, Y-84, deep inside the Asteroid Belt. Only go there when you have defeated all three planets (Jungle, Ice, Fire) and have all three Space Knights with you. Only then can you draw the Golden Key from the crystal.
Travel to the giant asteroid at X-11 Y-82 to battle the Bard and save the Princess. You will need all three knights to pull the Golden Key and reach the other galaxy.