Ken's answer is basically correct. the system base for Zagyg's Castle will be the new C&C rules from Troll Lord Games. The original spirit of the material can bets be presented in a system that is close of O/AD&D, of course.
I am in process of completing a very large campaign base module for the project, this likely appearing as a boxed set titled Zagyg's Castle, Yggsburgh. The material in this covering the land around a town of some 10,000 inhabitants, the town, various smaller communities, and many encounters (five dungeon-like areas in the lot), adventure hooks for the GM to flesh out. the text covers all manner of background details for the GM, so as to set the area in an existing campaign world, and develop the work through exposition in playby the group adventuring therein. The GM will know the economics, politics, and society completely, can modify it easily and make it hos own.
As soon as that is published, the Trolls will follow up with the second part of the series, Dark Chareau, a module designed by Rob Kuntz that introuces the Mad Archmage laterally by revealing his former domicile, that inhabited before Zagyg became a more-than-human entity.
After the second part is released there will follow about six further modules, each successively detailing Castle Zagyg, its upper works and dungeon levels. This modular presentation also allows for expansion of the dungeons and addition of "side adventures" as was done in our original campaign, examples of such add-ons being like my modules Dungeonland and Isle of the Ape.
In all, we expect to be busy completing this extensive creative project over the next two years.