A castle or palace.
There are a number of words for castles and other great houses or fortified compounds – keep, citadel, fortress, stronghold, manor, estate, manse, etc. Sometimes, though, you may want to invoke a certain exoticism or foreignness that such words lack. “Schloss” is the German word for castle and provides for this quality.
Going back a little bit further, schloss comes from the Old German sloz (also “castle”). It may be worth noting that Wiktionary’s entry specifies a schloss to be an unfortified castle or chateau. But if you’re using it for a fantasy world, I’d imagine it’s ok to be fast and loose with this designation!
I don’t know how likely you are to come across the term today, but you may encounter it when reading older stories.
In Styria, we, though by no mean magnificent people, inhabit a castle, or schloss. A small income, in that part of the world, goes a great way. Eight or nine hundred a year does wonders. Scantily enough ours would have answered among wealthy people at home. My father is English, and I beat and English name, although I never saw England.
– Joseph Sheridan Le Fanu, Carmilla (1872)