Q. about protective dome spells

[Crazy]

I took over a province with a castle. Every few turns I get attacked by huge armies.

Before trying to break siege I send several "Fire from Afar" at the sieging forces.

Last turn I cast "Horde from Hell" at a sieging army and I get a message that the spell was destroyed by a dome protecting the province.

But it is my province!?!

I certainly did not cast any dome spell. How would I check if one is active.

Is this a bug??