"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

I was methodically destroying everyone. No problems with capturing castles. Everything on the first try. Now, only one castle with a Biaara remains. I read the forum – it's scary. I think I'll wait and save up. Maybe I'll even see a UFO.

Can you tell me what it means to upgrade a СА (Castle)?
What is this СА thing?