What is the meaning of Fortress?

A fortified place; a large and permanent fortification, sometimes including a town; for example a fort, a castle; a stronghold; a place of defense or security.

A position that, if obtained by the weaker side, will prevent penetration by the opposing side, generally achieving a draw.

To furnish with a fortress or with fortresses; to guard, to fortify.

Source: wiktionary.org