Using DCCRPG-style spell checks in Castles & Crusades requires some careful integration of the Siege Engine with corruption and spell misfiring. Spell levels are very different between the two games (DCCRPG ranges from 1-5, while C&C has spells ranging from 0-9), but in the end the math pretty much all works out.
To Cast a Spell: Roll a d20. Add the character's level and relevant attribute modifier to the result. Subtract the level of the spell. Add in any other bonuses or penalties. The sum must be 12 or higher for the spell to be cast. Consult the following table for results.
Spell Check Sum
Spell failed and lost for the day. Roll on spell misfire and relevant corruption table.
Spell failed and lost for the day. Roll on spell misfire table.
Spell failed and lost for the day.
Spell failed but not lost.
Spell cast as per spell description.
Spell cast with additional beneficial effects as per GM’s discretion.
Spell levels correspond to which corruption table is rolled. Levels 0-3 roll on the minor corruption table. Levels 4-6 roll on the major corruption table. Levels 7-9 roll on the greater corruption table.
While this solution doesn't do much to add all of the flavor that comes from DCCRPG magic, it does add some danger to the hundreds of C&C compatible spells and dumps the fire-and-forget rules that can make C&C magic extremely dull at low levels.