Castles & Crusades is one of those games that is "old school" in flavor without trying to reimage any of the specific classic D&D rulesets. It certainly follows AD&D most closely, but even with that, many of the classes are rewritten in a way that they don't mirror the source material too closely. The secret, I think, is the "flavor".
C&C feels and tastes and even looks very much like AD&D without being AD&D. I've played in a long lasting C&C campaign online, and it felt very much like AD&D despite all the changes. My biggest gripe, and the toughest nut for me to crack, is the Siege Engine.
The Siege Engine is pretty much a universal skill check system for C&C and it's also the one piece that takes me out of my AD&D feel. I just don't like it. I feel like it was thrown in to give the system something that wasn't part of the OGL for marketing and copy write reasons.
Still, it is probably the best supported of the Old School rulesets, with a large assortment of modules and adventures for use with the Fantasy Grounds 2 VTT. Actually, with the exception of the LL rules, C&C is the only Old School ruleset available for FG2.
The best thing about the rules? From OD&D to 3.5e, I can run a module on the fly. Probably could with all the OSR rulesets for everything up to 2e, but 3e and 3.5e get a bit shaky ;)
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