Instructions
Get the bag of manure from the hooded guy, and power up the hovercraft. Then complete the star puzzle in the Princess' Tower. This opens the secret chamber with a key. Exit the tower, and head to the fountain between the tower and the castle.
Enter Mordred's Museum -- just to the left of the fountain. Go to the top floor, and click underneath Mordred's bed to get a note and coordinates to Pewter Moon. Return to the stables at the castle, with the hooded guy who gave you the bag of manure.
Push the two bales of hay by the hooded guy, to reveal a locked gate on the floor of the stable. Use the key you got from the secret room in the Princess' Tower to open this gate. Then use the rope to slide down to the bottom, where you'll find an owl.
Follow the owl back up, then release the mechanical mouse from your backpack. The owl will eat the mouse and join your party. Go back down the hole, and push against the wall on the left until it crumbles. When you enter the room, the robot in the cell will self-destruct, leaving behind a fuel rod. This is the "manure" Mordred refers to in his journal that he used to get to Pewter Moon. Click on the owl, then click on the fuel rod to fly into the cell and collect it.