The workshop of Mordred is hidden in an underground chamber, its entrance under two hay bales on the mill farm. You will need a key to open it. (see related question)
There are two books in the library and one in Mordred's laboratory.
You do not get or use a bucket on Astro Knights Island.
There is no treasure chest on Astro Knights Island.
There are no bird eggs in astro knights island.
its on the island
In Mordred's underground laboratory, the wall to the left is just dirt and can be pushed down.
How do you talk to the girl in the hay loft at astro knights island?
Go to the hay bales and push them apart. The key opens the hatch to Mordred's underground laboratory.
On every island you have to be the one to save whatever needs saving. That includes the kingdom in Astro Knights Island.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
The key doesn't go to the saucer. It goes to Mordred's secret laboratory. (see related question)
It unlocks Mordred's secret laboratory. The entrance is between the two hay bales in the farm area near the mill.