If you already have the De'Arnise keep as your stronghold, I think that's it. Unless I missed something, you can have only one stronghold. De'Arnise comes pretty early on; it's a cash cow but the SH quests afterward are only moderately interesting.
Your character would have had to decline Nalia's offer for De'Arnise to be able to get the Priest stronghold or the Mage stronghold.
