Castles & Crusades is a fantasy RPG by Troll Lord Games. It uses what they call the Siege Engine™, which is a d20 system very close to AD&D but with a ‘modern’ mechanic (ascending armor class for example). If you know AD&D, learning C&C is very easy since all the classes and all the spells are basically the same. There are differences, for sure, and you should read the spells to see what changed, but you’ll find most of what you’re used to (Magic Missles, Fireball, Barkskin, etc.).
One of the key differences is the concept of prime attributes and the system for attribute checks. To make an attribute check you roll 1D20, add your characteristic’s modifier and usually your level and compare that to a difficulty class (DC) which is often ajusted by the level of what you’re up against. The base DC is 18 if it’s not a prime attribute, and 12 if it is. Each class has it’s own prime attribute, and the player can choose a second one (humans get to choose 2, for a total of 3 versus a total of two for other races1).
The system is well supported, with a full range of rulebooks (Players handbook, Monster & Treasures, and Castle Keepers Guide) and modules and supplements (Runelord, Aufstrag campaign, etc.) and many settings (Aihrde, Inzae, Haunted Highlands, Celtic Adventures).
which is a significant advantage and makes human characters more interesting than in many systems. ↩