fortress
A military building or walled compound with large defensive fortifications.
A fortress is a walled construction with very heavy fortifications around its perimeter or on an exposed side, which was built for military or defensive purposes. It typically features thick masonry or concrete walls, a jagged perimeter, towers or castellations at intermittent points, and very narrow window openings for defense by archers or riflemen.
Fortified buildings constructed mainly or in part as permanent residences, especially for aristocratic or royal persons, are classified as castles rather than fortresses.
The original purpose is military or defensive. A fortified permanent aristocratic residence has the separate castle meaning, even where the defensive architectural elements look similar.
