The portal is the connection to the "distant galaxy" where the Binary Bard has his castle. It is sealed with the Golden Key on an asteroid in the asteroid belt, near X-11 Y-84. Once you have beaten the three planets (jungle, ice, fire) and have all three space knights with you, you can go there draw out the Golden Key and enter the portal, to find the Bard and the kidnapped Princess.
You do not get or use a bucket on Astro Knights Island.
There is no treasure chest on Astro Knights Island.
You go through the puzzle door and beat Mordred. (see two related questions)
There are no bird eggs in astro knights island.
its on the island
How do you talk to the girl in the hay loft at astro knights island?
On every island you have to be the one to save whatever needs saving. That includes the kingdom in Astro Knights Island.
The portal to the castle where the Princess is held is on a planetoid in the asteroid belt, near X-11 Y-84.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
on astro knights island
With Mordred
Astro Knights