On March 25th, 1999, a Teaser demo for Return to Dark Castle was released, it included only 1 level, with no enemies, exits, or anything to do but jump around.
A few years later, Dark Castle fan Jon God was hacking through the data, and ran across a lot of resources that were not used in the teaser.
The Teaser included many sounds, and images that are not found in the standard Teaser level, Day & Night.
On closer inspection there was also, 18 levels, only one of which appeared in the Teaser
Main Article List of Levels in the Return to Dark Castle Teaser
There are 18 levels in the Return to Dark Castle Teaser, some of which are completely pointless, others seem near completion.
The Teaser only runs in Mac OS 9, so to run on it on a computer of today, you will need to setup SheepShaver.