The bedroom for the Princess is in the left tower of the castle, which is now cut off from the main building. To get to the tower :
To catch the mechanical mouse, in the Princess's Maid's room (castle first floor left).
Nobody know's excatly what it is except for the creators that's why its called "Secret"
You have to get the note in her chest, that is in the far end of the room. Here is my guide:http://wiki.answers.com/Q/How_do_you_win_astro_knights&alreadyAsked=1
You show her a secret message from the Princess. You find it in the Maid's room (castle first floor left).
You need the Princess's secret message from the maid's room (castle first floor left).
There are two messages: the Princess note in the Maid's room (castle first floor left) and the note in the Princess's room in the separate tower. Both should go into your item bag.
The Princess's Maid's room has the secret message and the mouse. It is on the first floor of the castle, to the left. The Princess has a room in the tower that is now unreachable. Get the rope from the windmill's water wheel and tie it to one of the big arrows on the giant crossbow. Shoot it up and left and you can hit the tower door.
The Maid to the Princess has a room in the castle (first floor, to the left), and there is a secret message and a mechanical mouse there. However, the Princess has a room in the left side tower, which is cut off from the rest. You can get the note that is there : tie a rope to one of the big arrows and shoot it that way.
If you have the Secret Message from the Princess (from the maid's room in the castle), the mill girl will give you the password to the secret room below the fountain. (see related question)
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)
well if you go into the princess room you will see a draw open it and get out the note.
In the princesses bed room