I was recently told that my post on how the Siege engine that powers Castles and Crusades, Amazing Adventures, and Harvesters works is too "wordy". So what follows is my simple "how it works".
Difficulty of rolls:
At character creation some attributes are designated primary and others secondary.
Rolls against a primary attribute start at a difficulty 12, secondary are difficulty 18. This is the base complexity class (CC).
The GM then applies any complications as a complexity level. For example Dex (primary 12) roll for picking a CL4 lock would be (CC) difficulty 16.
How to make an attribute roll:
First the appropriate attribute is chosen for the roll. The player rolls a D20 and adds the modifier for the attribute. If the roll being made is either covered by a class ability, knowledge, or action appropriate to the class add the characters level to the roll as well. Thats it.
Base CC + CL
Just as added examples, if you were to get stung by a giant scorpion, and it was HD 3. 3 would be the CL for the constitution save.