The sorceror stronghold is the same as the mage's - the planar sphere. The quest to get it - go to the government building, talk to the cowled wizard there (name starts with T, can't remember it
), who'll send you to get Valygar.
Once the stronghold is yours you'll get three apprentices who will make you magical items. If you do it right and they don't die, one will come back after they've graduated and make you five potions a week. Plus experience for helping them etc.