This is a post I have wanted to do for a while. Mostly so that I can link to it when trying to explain how the siege engine works and why I like it.
First off the game itself is an OGL based game. That emulates the feel of the first edition of AD&D. As I like to say, Its 1E with 3E math.
Where the siege engine comes in is the choosing of "primes". Some ability scores are prime while others are secondary. 3 primes for humans. 2 for other races to balance racial abilities.
One prime is chosen by your choice of class. Others are chosen by the player. The difficulty of almost any given roll will be based on the ability score it is based on. If the ability is secondary the difficulty base will be 18. If the ability score is primary the base difficulty will be 12.
Beyond mechanics primes/secondary choices are also a great layer of customization. For example a thief with a primary of charisma could be a smooth operating, fast talking, confidence man. While a thief with strength as a prime could be a thug, relying on force and muscle to "get the job done".
Next comes class skills, a lot of people seem to be under he impression that Castles & Crusades does not have skills. Rather Castles and Crusades focuses more on what skills your class excels at, but doesn't limit the character to what they can attempt.
Since a characters level is the measure of prowess in his chosen class, level is added to all rolls for class skills. Also skills are linked to an ability. For example Pick lock (Dex). Helping to quickly identify which class the skill is based on.
Lastly is the complexity level. This is a modifier to the difficulty base. This number is chosen by the CK or based on the action attempted.
Ill give a few example rolls to help show how all this comes together to make a fast and intuitive system.
For example, A level 4 Human Rogue with primes of Dex (+1), Con, Char (+1), Secondary of Int, Wis+2, Cha is attempting a few skill rolls.
Attempting to pick a lock: For the Rogue this is a class skill and a Dex based skill, which is prime for the Rogue. The lock is a moderate difficult lock CL 3. So the final difficulty is 15 (12 for prime +CL3).
The player would roll a d20+ 4(his level)+1(Dex ability modifier) and try to beat the 15 difficulty.
Attempting to track an enemy: For the Rogue this not a class skill and is a Wis based skill, which is secondary ability for the Rogue. The tracks are fresh and a fairly easy read CL 0. So the final difficulty is 18 (18 for seconary +CL0). The player would roll a d20+ (no level added, not a class skill)+2(Wis ability modifier) and try to beat the 18 difficulty.
Attempting to decipher script: For the Rogue this a class skill and is an Int based skill, which is secondary Ability for the Rogue. The script is somwhat familiar to the Rogue so its fairly easy CL 1. The final difficulty is 19 (18 for secondary +CL1). The player would roll a d20+ (+4 level added, it is a class skill)+0(Int ability modifier) and try to beat the 19 difficulty.
Also as a note monsters use number of hit die as level and are designated primes as "physical, Mental, Or Both". With stat blocks like that of 1E AD&D. Making most "old school" monsters a snap to use.
Another note is that Amazing Adventures is a Pulp Era game that also uses the Siege engine. Instead of the 12/18 split for prime/secondary AA uses a single Difficulty of 15. With a +5 bonus for primary.
Because so much of the game is OGL based. And at the same time draws so much inspiration from previous editions of D&D it is very easy to use what you like from these other games. If you like feats you can add them. Like secondary skills or proficiency slots? use them too.
As far as I can tell Troll lord games was the first to use the OGL to emulate an older style of D&D experience. First by just stripping down the system and using it as a simpler form of D20. These stripped down adventures and modual came out at the same Gen Con that 3rd edition of Dungeons and Dragons was revealed. Later with the addition of the siege roll and its own players handbook the game took on a life of its own. While others used the OGL to replicate the rules of older editions or D&D, C&C used the OGL to replicate the feel of AD&D while leaving most of the rules clearly d20. And it really has taken on a life of its own. C&C has grown and changed, Their products continue to improve and grow. To many of us C&C is no longer an emulator, But rather a great game in its own right.
List of games that use the Siege Engine:
Castles and Crusades: This is the fantasy game with 1E sensibility and 3E math.
Amazing Adventures: A pulp era game using a variation of the siege engine.
Harvesters: This is a game about anthropomorphic animals. Its uses a race as class vary close to an older edition of D&D.
Star Siege: this is a sci-fi game, At its core it is a siege game, But it is not a level based system. Its point buy, from character to gear its all customizable.