Hello, my name is Chris Rutkowsky and I did the conversion of the Mysterious Tower from d20 to Castles and Crusades, and wrote an original module for C&C for Goodman Games.
Joe forwarded me an email in which a person wanted a guideline for converting a certain module (Crypt of the Devil Litch) to C&C. I personally don't have this module, however I can give you guys a quick guideline that I used myself to make things for Mysterious Tower work for C&C.
The easiest thing to convert is the monsters. All you need to do is look up the same monster in the C&C Monsters and Treasure book! You can mark the pages in the module and your M&T book with color coded sticky notes so that you always know what page to turn to in a hurry and don't have to bother with making notes.
If the monster you want to convert is not in the Monsters and Treasure Book, I would consult an older edition of D&D-- my personal favorite being the Basic D&D Rules Cyclopedia. If you don't have it you can buy the pdf for 5 dollars at rpgnow.com or buy a used one on ebay.
The monsters presented in the RC are 90% compatable with C&C rules. The only thing that you would need to actively convert is armor class and saving throws. To convert D&DRC armor class to C&C, simply subtract the given AC from 19. Remember that subtracting a negative number is 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). Simple, eh?
If using 1st or 2nd Edition AD&D as a resource, armor 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. Saves convert the same as in basic D&D.
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. So a creature listed as doing d4+5 damage, change it to 2d4. If a creature had d6+3 dmg, 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. 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.
Older editions of D&D had 5 saving throws. C&C has 6 and they are just about 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:
Breath Weapon (and area of affect spells like fireball)-- Dex
Poison, Death-- Con
Wands, Staves, Wands (and Illusions)-- INT
Spells (except illusions and area of affect spells like fireball and charm or sleep spells)-- WIS
Enchantment spells (sleep, charm, etc) and fear= CHA
Converting d20 saves is a little trickier and involves a judgement call by the CK converting it. You just have to use some logic as to which converts to what.
FORT-- STR or CON (depending on the effect)
WILL--INT, WIS, or CHA (Depending on the source of the spell/effect, CK'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 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).
Converting damage from traps--
If the trap is related to a spell, look up the C&C equivilent and adjust the effect accordingly.
If the 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, it just does 5d6.
Well, that is all that comes to mind for now. Let me know if you think of any other questions about conversion.