Crumble bugged?

[Xietor]

It does more if the mage has a higher level of earth magic then the spell requires, also you add the str of the attacking force squared and divided by 100 to that number minus the str squared divided by 100 of your defenders.

But you said he was an e5 mage. shrug.

Maybe his defending troops died of disease, or fled the castle(stealth). Or some other event killed his defenders.