The Siege Engine works on an attribute check system. A character's attributes are divided into primary and secondary attributes. Checks made against primary attributes have a base Challenge Base (or target number; abbreviated "CB") of 12, while secondary attributes have a CB of 18. The game's referee, the Castle Keeper, adds a challenge level (usually from 1-10, depending on task difficulty) to the CB and the resulting number, the challenge class or CC, is the final target number required to succeed at a check. The player adds the character's level, any attribute bonuses and class bonuses to the roll of a twenty-sided die. If the result after bonuses equals or exceeds the challenge class, the player succeeds. Except for combat, the Siege Engine is used for anything that requires a check in the game.