(There are several answers at the related questions below)
Into the castle. To follow him, you have to solve the puzzle door. (see the related question)
Solve the door puzzle and battle Mordred to save the (real) Princess. (see related questions)
You can see the rest of the Poptropica solar system, including the nearby moon. This is only information and nothing you need to solve the game.
Solve the door puzzle (he's Mordred) and battle his robot warrior suit. (see related questions)
Solve the puzzle door and enter to fight Mordred's robot suit, first as Merlin then as your character. (see related question)
The solution to Astro Knights Island is at the related question below. Your quest is to rescue the Princess, who has been kidnapped by robot invaders and taken into space. Find and repair the saucer Excalibur and launch into space, to find the three planets where space knights are searching for the princess. Defeat the robot monsters on the planets using the Mystical Weapons of Arturus, then face the Binary Bard in his castle in another galaxy. (see related solution question)
To solve the puzzle, you have to press 10 of the panels exactly once each.They are :All four cards in the 2nd rowThe two middle cards of the 3rd rowAll four cards in the 4th row.
You can click on the door behind "her", but if you give her the weapons, you have to solve the door puzzle to enter the final battle against Mordred. (see the related question for the solution)
On the secret planetoid (X11 Y82), you can draw the Golden Key from the crystal, opening the path to the castle in a far galaxy. To enter, you will have to solve the door puzzle by the Binary Bard. (see related question)
Click everything in the 2 row, the 2 middle ones in the 3, and everything in the 4
To enter Mordred's castle, solve the door puzzle. Click once on each of the 4 cards in the second row, the two middle cards in the 3rd row, and the 4 cards in the 4th row.
Your supposed to solve the puzzle on the door. When it reveals the joker persons picture you have to go inside and defeat his robot. Its hard to explain how to beat him so just look it up on youtube.