IF developer/author Andrew Plotkin made a puzzle game in 1989
using just directory structures:
It was originally a bunch of files and directories living in a shared Unix filesystem at CMU. You'd look around by typing ls; the room description was just a blank file with a long name! The room showed you puzzles, and you could enter answers by running a program which lived in that directory. If you answered correctly, the program would notify me and I'd add you to the access list for the next directory. Yes, manually. It was just my friends at CMU.
That's incredibly elegant! He also writes about an interesting server-side (because that was the only side you could do this on in 1995) database search app for finding verses from an extremely long song about religions. Wonderfully weird.