Fortress (noun)

1. A large, strongly built structure, typically of stone or brick, used as a defensive stronghold.
2. A place, condition, or attitude characterized by strong resistance to change or attack.

Origin:

Middle english: from old french forteresse, from latin fortis "strong" + -ressus (suffix indicating possession).

Examples:

1. The fortress stood on a hill overlooking the city.
2. The fortress was thought to be impregnable.
3. The company's stronghold in the market is a fortress.
4. He built a fortress around his heart to protect himself from pain.
5. The fortress of her beliefs was unshakable.