I remember having a lot of fun with that back in the day, and looks like the old gem still got it.
Surely there are several aspects which wouldn't impress today, but one has to respect the breakthogh it represented 20+ years ago.

And the number one reason it's still great fun is its classic Thief gameplay - remaining stealthy, avoiding guards, robbing a big lavish mansion blind, while graudally uncovering the stories and secrets of its inhabitants. You can't ask anything more, and The 7th Crystal delivers more than you would ask of a 2001 FM.

*And if you're in for some unconventional entertainment, this mission offers abundance of ways to have fun with guards. Locking the whole level away in the attic was my personal bonus challenge.