Here are links to the basic info you need to play in Ran Lithby. In general the rules are original OD&D.
Character Creation
House Rules
The general background:
Ran Lithby, hundreds of years ago, was a powerful wizard and hero of renown. He retired to the mountains with his riches and followers and built a modest castle and outpost in a small valley. Legions of stonecrafters, artisans, and laborers marched into the valley but the castle and outlying buildings remained the same: it became apparent that Ran Lithby was building his fortress deep into the mountains. Years passed and the excavation continued. Reports from the site became weird, returning workers reported long underground marches through a veritable city before they got to new excavations, odder assemblages of hirelings were seen entering the works, weird and dangerous creatures were found wandering the mountains nearby. Ran Lithby himself was not seen in pubic, ever. Then, over a course of several years, the flow of money out and workers in stopped. Several decades passed and no one heard anything at all from the mountain. Ran Lithby and what had come to be called his Folly were forgotten by most.
But not by all. Tales began to spring up all over the land - always about the woodcutter's nephew, or the miller's uncle, or that old carpenter from two villages over - of those who had journeyed to the Folly, and come back with strange tales and rich goods. And tales of those who boasted they were going to the Folly and were never heard from again.
Now the Folly is one of the places that you go when you are ambitious, disaffected, or seeking adventure. A steady stream of fortune-seekers and the curious find their way to the nearby village of Inkwell to stage an expedition…