If the internal cd-i date is set to any Friday the 13th, a horror theme will be present in the Great Hall at the beginning of the game. Likewise if the date is set to 25 December, a Christmas tree will be present in the Great Hall.
While in the Great Hall at the beginning of the game, press Down, Down, Down, Up, Up, Up, Down, and Down. At the bottom of the screen you'll be able to access any room in the castle by selecting the numbers that appear.