You go through the puzzle door and beat Mordred.
(see two related questions)
To see the solar system on Astro Knights island, go to the Planetarium. (not part of the game, but an interesting view)
Go on youtube
you go into the portal. then you go to the bottom and talk to the princess.
When you defeat all the planets in astro knights island you go to the coordinates X-12, Y-83 ten there should be a giant asteroid retrieve the key from that planet and a portal will appear go in the portal and you will see the "princess" but it's a trick it's really Mordred. HOPE THAT ANSWERS YOUR QUESTION
You can go to space on Astro Knights
go to the secret door
The knights tell of a secret portal on a giant asteroid. The planetoid is hidden in the asteroid belt at the lower left of the solar system, near X-11 Y-82. That is where the Binary Bard has taken the Princess.
On the Poptropica website, all of the available islands are displayed on the Island Map.
His castle is in a far galaxy, connected by the portal of the Golden Key. When you have defeated all three planets (fire, ice, jungle) and have all three knights, go to the Giant Asteroid at X-11 Y-84 to reach the portal. (see related question)
you run into it and push then there you go
No, because you need the three knights to go with you. (see related question)
In order to get the coin from the fountain in Poptropica Astro Knights Island, you must go to the fountain and look for the shiny coin. Once you spot it, you walk over to it and it will be added to your dock.