Taken from the last page of the Manual:

Top Secret is an area you can access only once you’ve completed the main game. The sequel to the game will give you a password. Use the password in the Top Secret area labeled ‘The Hidden Door’ to gain access to a rare bonus.

The password mentioned above will be revealed in the sequel to Professor Layton and the Curious Village. For detailed instructions on how to retrieve the password, please refer to the manual for the next game in the Layton series.