The Princess's secret message is in the Maid's room (castle first floor left). This will allow you to get the Password from the girl in the mill.
The other note by the Princess is in her bedroom (the unreachable tower on the right side of the castle), That one tells you to look under Mordred's bed.
(see related question)
It is on tp of the castle roof in astro knight island
The castle is the second area on the island, to the right of the Village.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
You show her the message that you get from the castle (maid's room, first floor left).
Walk to the right.
its on top of the castle
You show her a secret message from the Princess. You find it in the Maid's room (castle first floor left).
There is no jail on Astro Knights Island. There is a castle dungeon, where an attacking alien robot is being held after his capture. (see the related question)
On the top of the castle
There is a castle on Astro Knights Island, in the kingdom of Arturus.
The secret message from the Princess is in her Maid's room (castle, first floor left). There is another note by the Princess in the left tower which is now disconnected from the castle. (see related question)
You need the Princess's secret message from the maid's room (castle first floor left).