"Castles & Crusades"s game mechanics are based on the d20 system, designed by Wizards of the Coast. The system has been modified to create a simplified version of the game. All the core classes and races, the alignment system, attributes and hit points systems were retained with only slight adjustments in hit dice. The highly intricate system of skills and feats found in "Dungeons & Dragons" 3rd edition was discarded, replaced by what the designers call the "Siege Engine", intended as an extremely easy game mechanic with universal applications. The game is compliant with the terms of the Open Game License. The game was later nominated for a 2005 ENnie Award for Best d20 Game.