For those of you who doubt it as easy as people claim, or want a formulaic way of converting Monsters and Scenarios to Castles and Crusades - presented here are some guidelines.
These ideas were originally posted on the Goodman Games Forum way back in 2006 by Chris Rutkowsky (who did the conversion of the Mysterious Tower from d20 to Castles and Crusades, and has written an original module using the Castles and Crusades system for Goodman Games).
If the monster you want to convert is not in the Monsters and Treasure Book, the easiest way is to consult your AD&D/D&D Monster Manuals and find the Creatures entry in there (or the NEAREST suitable entry) and use those stats.
But for those of you who want a more calculated approach -
Converting from D&D/D&D.
To convert a D&D Armour Class to C&C, simply subtract the given AC from 19. Remember that subtracting a negative number equates to the same thing as addition.
So a D&D monster listed as AC 5 would be AC 14 in C&C (19-5= 14). A monster with AC -3 would become AC 22 in C&C (19+3 =22). Saves are easier to convert. If the creature saves as a fighter, thief, dwarf, or halfling it is Physical prime. If it saves as an elf, cleric, or wizard it is mental prime. Use the creatures HD as the bonus to all rolls (including saves, attacks, etc).
If using 1st or 2nd Edition AD&D as a resource, Armour Class is also easy to manage. Simply subtract the listed AC from 20. So an AC 5 creature would be AC 15 in C&C and an AC -3 character would be AC 23 in C&C.
Converting Save Throws
Older editions of D&D had 5 saving throws. C&C has 6 and they are roughly the same, except the C&C saves are directly linked to stats. The old D&D saves were as follows and converts to C&C as presented below:
Saves vs. Paralysis – Castles & Crusades Strength Based Saves
Saves vs. Breath Weapon (and area of affect spells like fireball) – Castles & Crusades Dexterity Based Saves
Saves vs. Poison, Death – Castles & Crusades Constitution Based Saves
Saves vs. Wands, Staves, Wands (and Illusions) – Castles & Crusades Intelligence Based Saves
Saves vs. Spells (except illusions and area of affect spells like fireball and charm or sleep spells) – Castles & Crusades Wisdom Based Saves
Saves vs. Enchantment spells (sleep, charm, etc) and fear – Castles & Crusades Charisma Based Saves
Converting damage from Traps.
If the Traps Damage is related to a spell, look up the C&C equivalent and adjust the effect accordingly.
If the Traps Damage is related to a weapon, look up that weapons damage in C&C and convert it.
If damage is listed as a die with a damage bonus exceeding the die types, add another die to the damage instead. If it has a damage bonus less than the die type, get rid of the bonus damage.
So if a trap is listed as doing 5d6+7 damage, convert it to 6d6 damage for C&C. If it does 5d6+4 damage, its damage is just does 5d6.
Converting from the D20 System.
The Creatures/Monsters in D&D 3rd/3.5 scenarios are 90% compatible with C&C rules. A lot of the work can simply be done as you go along "on the fly" so to speak.
If you want to convert DIRECTLY from d20 system, use the same Hit Dice and die type as the creature already has, but get rid of the bonus hit points.
So a creature listed as 5d10+15 HD in d20 would convert to 5d10 in C&C.
You should also take away any bonuses to the damage dice listed.
If the bonus listed is higher than the die, just make the attack use the die type, and double it.
Therefore a creature listed as doing d4+5 damage - change it to 2d4. If a creature had d6+3 damage - just convert it to d6.
These changes are because hit points and damage are harder to come by in C&C than they are in d20.
D20 System Saving Throws.
If a creature's good saves are FORT or REF, it is Physical prime. If it is WILL the creature is mental prime. If they are all good saves, it is physical and mental prime.
Converting Character D20 saves is a little trickier and involves a judgement call by the DM converting it. You just have to use some logic as to which converts to what.
FORT - STR or CON (depending on the effect)
REF - DEX
WILL - INT, WIS, or CHA (Depending on the source of the spell/effect, DM's call)
Converting DCs to Challenge Levels.
I use a simple formula for this. I assume that DC 15 is the average DC of a check in d20. In C&C, the average difficulty of a task is CL 0. So every 1 the DC is higher than 15 in D20, the CL is 1 higher. So a DC 19 check would convert to a CL 4 check. For every 1 the DC is lower than 15 in D&D, the CL is -1. So a DC 12 check would be CL -3 in C&C. The CL of course is added to the base of 12/18 depending on if the character is prime in the required stat.
Converting D20 System Skill Checks .
Simply make the skill check into an attribute check for the skill that is normally tied to that attribute. You may want to give certain classes a bonus in this, or even restrict who can try based on class. For instance a Search check would simply convert to an INT check, which anybody can do, but a Survival check to track somebody should be limited to rangers or maybe druids (with a penalty as it is not a class ability for them).