To which version of the game does this apply?  Untested.)

This material was exported from the Codebook 4:
http://www.codebook.pp.se/

Dark Castle : Easter eggs

Surprise

Set your computer date to friday 13th, load Dark Castle 
and get a pleasant surprise.


Christmas

Set the date on your clock to December 25th using the 'Date & Time' control panel. There should be a few differences in the hall.

