There are two solutions here: one deals predominantly with player actions; the other deals with the ramifications of the game scenario. Both are solutions providing "spoilers."
In addition, the Poptropica site offers hints for the game.Solution #1: (Solution #2 below includes additional details in the same order)
The Kingdom Arturus on Poptropica (boldface are collectible items)
* You will be wanting to know where these coordinates came from. First, grab the rope on the wheel mill in the swamp. (You may have already grabbed it.) Next, head right until you reach the castle. Climb up until you see the crossbow. Attach the rope to an arrow and shoot roughly to the upper left. It might take several times, but eventually it will stick in the door of the princess's room. Walk on the rope and enter the princess's room. Once in the princess's room, pick up the Princess's Note. Once you have this, exit her room and drop down to the ground, heading left. Go back to the Mordred exhibit and search under his bed. You will find the Missing Page from Mordred's Journal. At the very bottom of this page are the coordinates 56, 52.
Space, above Pewter Moon
The Final Battle
YOU ARE AWARDED THE ASTRO KNIGHTS ISLAND MEDALLION
Solution #2 (includes more detail):
In the Kingdom of Arturus
On the Pewter Moon
Traveling in space
The Jungle Planet X-15 Y-15
The Ice Planet X-73 Y-83
The Fire Planet X-83 Y-20
The Secret Asteroid X-11 Y-84 (in the asteroid belt)
The spaceship is parked on the castle (as whenever you may have returned during the game), and you can fly to the planets or shoot asteroids and aliens. However, without the Orb, Mordred's planetoid is dark and the castle inaccessible.
(end Solution #2)Read more: http://wiki.answers.com/Q/How_do_you_beat_astro_knights_island#ixzz1ejIp9diY