No longer using AC in castles and crusades.

I really like how the siege engine works in C&C. Why not use the siege engine for combat too? Rolling to hit would be simply rolling on whatever stat you would roll to attack on normally for a siege check (Str for melee, Dex for ranged). And armor bonuses would add to the difficulty of the roll.

So for example if your strength was prime your target number to hit with a broad sword would be 12. If the target was well aware of the attack (Dex bonus of +1) and wearing leather armor (+2) the difficulty to hit him would be a 15.  (this is not entirely a new idea, a Variation is used in starsiege.)

This option may work even better in a setting where armor is damage reduction rather than an armor class bonus. Making difficulties to hit just your base difficulty and dex bonus. (as it is in star siege.)

Another option to sprout off of that last one would be to do both. Armor adds to difficulty to hit. And also gives a damage reduction based on how heavy it is. 1 point for light armor, 2 for medium, and 3 for heavy.

This could also be split into two ways too. Regular armor bonus adding to difficulty to hit. While magic defense bonuses are damage reduction. Or switch that and make magic make you hard to hit and magic is damage reduction.

Remember, Now that an attack roll is a siege check add your level to attack rolls.

Just some options to toss around.