Essentially, Castles&Crusades is a D20 system. Roll high. Roll higher than your opponents armor class, you hit. Roll higher than the challenge class of a task (same as a difficulty class in other games) you succeed. Roll higher than the person/thing attacking you with your saving throws, you succeed. Basic D&D 3rd edition. Very similar to Dungeon Crawl Classics.
The workhorse of the Castles and Crusades game is called the Siege Engine.
Challenge Class (same as difficulty class): the number you need to beat by rolling high on a d20.
Challenge Base: the number you have to beat based on which attribute you are using.
Challenge Level: set by the game master. It is the number I add to the challenge class (see below).
1. Based on your race and class, you will have attributes that are either primary or secondary.
2. If your attribute is primary, you have a challenge base of 12. That means in order to be successful in any challenge that you face using that attribute, you need to roll higher than a 12.
3. If your attribute is secondary, you have a challenge base of 18. That means that in order to be successful in any challenge that you face using that attribute, you need to roll higher than an 18.
4. When you roll, you add your level and any modifiers related to that attribute. If you beat the challenge class, you are successful.
5. As the game master, I will add a challenge level to most challenge class numbers. This is a gray area that I can use my judgement about, but there is definitely some guidance on how to do this.
7. Thus: Challenge Class =s Challenge Base + Challenge Level (CC=s CB+CL)
6. Saving throws are determined by attributes. If your attribute is primary, your "to beat" number begins at 12. If it is secondary, your "to beat" number is 18. You add your level when you roll, as well as any modifiers related to the attribute.
Example: Thud the Barbarian is 5th level, and wants to pick up a boulder and throw it at an orc. How can he do this mechanically? First, we'd say the most important attribute in this action is strength. Since strength is a primary attribute for a barbarian, his challenge base starts at 12. Because Thud has an 18 strength, he has a +3 strength modifier. Because Thud is 5th level, he gets to add 5 to his roll. As the GM, I say this boulder is pretty big and unwieldy, so so I'm going to make the challenge level 3, making the challenge class for this action a 15 (challenge base 12 plus challenge level 3). Thus, Thud needs to be a 15 (challenge base 12 + challenge level 3). Thud rolls an 11. He adds his strength bonus (3) and his level (5), bringing him to a 19. Thud beats the challenge class of 15.
Example: Thud faces an illusionist who casts a powerful dwemer upon his furrowed brow. Thud needs to make a saving throw against Arcane Magic and Illusion. This save is based on intelligence, which is a secondary attribute for our poor barbarian. Thus, Thud's challenge base is 18. Thud has a low intelligence score, so he's at a minus one when he rolls, but he is also 5th level, so he gets to add 5. And, the illusionist is 5th level, which adds 5 to the challenge level. So Thud needs to beat a challenge class of 23 (challenge base of 18 + challenge level of 5) by rolling a d20-1. Thud rolls an 18, minus 1 for being dumb, but adds for for his level, for a total of 22. Thud falls for the illusionist's spell.
That's pretty much it. With regard to challenge level, anytime you wish to do a non-combat thing to a monster or an NPC, I can use their hit dice or their level to add to the challenge class. If Thud wants to wrestle an ogre to the ground, I might add the number of hit dice the ogre has to the challenge class. If Thud wants to disarm a trap, I might add the level of the thief who set the trap to the challenge class.
The Siege Engine basically replaces skills. Instead of making a History roll, or a perception roll, you decide which attribute is appropriate, determine the challenge base, and add a challenge level. Roll a d20, add modifiers, and level. You roll above the challenge class, and you are successful.
This ain't so hard to grok.