It's in the fountain next to the Mordred Museum. It is the shiny thing in the water about halfway up. Use it to pay your way into the Museum and find the Library Slip. (It is in a stack of books.)
the door that's locked
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.
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 the Golden Key and the Green Orb are returned to Arturus, there is no way to open the crystal gate.
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.
When you have beaten all three planets and have all 3 knights with you, travel to the giant asteroid at X-11 Y-84 in the asteroid belt. There you can pull out the Golden Key and enter the realm of the Binary Bard.
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 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.
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.
Yes, and the portal to the other galaxy is the Golden Key on the giant asteroid at X-11 Y-84 in the belt. You will need to beat all three planets (jungle, fire, ice) and have all three space knights with you to pull out the Golden Key.
To pull out the Golden Key, you must defeat all three planets (fire, ice, and jungle). (see the related question for the links)
YOU, the player, are the Chosen One when you arrive at the secret asteroid and must pull the Golden Key from the stone. If you don't have all three knights, you have wasted a trip. (See the related questions for help.)