Slitherine

I'm near the end of the game, actually, I think the game should have given me a victory a while ago. The next turn crashes on Horus's go. He was casting the spell of mastery until I took his capital. I wanted to win before he cast the spell. I'm not sure if the game allows an overall victory 70%/30% victory if the spell of mastery is being cast?
PS I think the problem is to do with me taking Horus's last city but leaving a settlement. I reran it so I destroyed the settlement first then took out the last city and it allowed me a victory.