[Taqwus]

It might have cast the spell twice, no?
To the OP, if you want to guard against spells, look into the 'dome' spells (Enchantment, mostly; Blood as well). Dome of Solid Air, Forest Dome, Dome of Arcane Warding try to block spells; the fire/water domes don't block spells but try to kill the casters; and the blood dome tries to both block spells AND punish casters. It does have its price, however...