Alternate Magic

This is a magic system I've been toying around with for a while now; this particular iteration is specific to Castles & Crusades and applies to all spellcasting classes:

You gain your normal number of spells whenever you are supposed to, Clerics and Druids still have access to all spells and must spend time each morning praying for spells. Wizards and Illusionists still can learn spells from scrolls. Instead of being "fire and forget" as standard C&C/D&D is, you now can cast any spell, any number of times. Your spell always goes off, but after each spell is cast you must make a Saving Throw using your relevant spellcasting ability (INT for Wizards and Illusionists, WIS for Clerics and Druids, etc.) base obstacle being 12 + level of spell, + the HD of creatures you affect. You roll 1d20 + ability modifier + level. If you succeed, continue as you were, if you fail, you take -2 Constitution damage (no HP damage, let's not make this more harmful or more complicated than it needs to be). If you lose all your CON this way, you die. You regain CON back at a rate of +2 per 8 hours of rest, so +6 per day. This damage cannot be alleviated via spells.