Craverly Heights, rather, a confusingly-similarly-named but completely unrelated game also by Ryan Veeder. I would never want to try porting Cragne Manor!
The original Colossal Cave Adventure has always been handwavingly treated as public domain, and nobody’s gotten in trouble for it, even when Microsoft released their own version for profit.
And now Zork I-III are released under the MIT license, which isn’t public domain but is very permissive.
Notably, though, all of these are parser games. I’ve previously proposed Mini-Cluedo as a similar small demo for choice-based systems (about on par with Cloak of Darkness for complexity; spec here), and there are versions in Dialog, Strand, Gruescript, Harlowe, Sugarcube, and Dendry for comparison.