1.
fortress, stronghold, fortification
A heavily fortified military structure or position designed for defense. Used both for historical castles and fortifications and in modern figurative contexts for impregnable places.
Usage
要塞 refers specifically to fortified military positions, as opposed to 城 (castle), which may serve as a residence. 要塞 emphasizes the defensive, strategic nature of the structure. The word appears frequently in historical, military, and gaming contexts. It can also be used figuratively to describe any extremely well-protected place.