"large fortress" is correct and usable in written English
You can use it whenever you are referring to a large structure that is designed as a defense and protection from attack or invasion. For example: "The massive walls of the large fortress surrounded the city, providing a shield against any invaders."
The city, an agricultural centre situated on the Silk Road and long famed for its large fortress, was designated a UNESCO World Heritage site in 2004.
The buildings are mostly blue with a huge mountain of stone crowned with a large fortress called Mehrangarh Fort towering over the skyline.
The once large fortress was the part of territorial and religious disputes between the Catholic Frankensteins and the Lutheran landgraves of Hesse-Darmstadt, the family in 1662 eventually sold their possessions around the Castle to the landgraves and retired to their possessions in the Wetterau.
From its origin as a small stronghold built by the ancient Illyrian tribe Dalmatae, becoming a royal castle that was the seat of many Croatian kings, to its final development as a large fortress during the Ottoman wars in Europe, Klis Fortress has guarded the frontier, being lost and re-conquered several times throughout its more-than-two-thousand-year-long history.
We had expected a large fortress-laboratory, but instead found only an 80ft-long creosoted garden shed.
The large fortress-like compound of the Abd al-Hadi Palace built in the 19th century is located in Qaryun.
The large fortress-like structures first started popping up around the world in the 1950s and continued through the mid 1970s.
Modern large fortresses like the Belgian stronghold of Eben Emael were nonexistent; the only modern fortification complex was that at Kornwerderzand, guarding the Afsluitdijk.
The Spanish crown decided to consolidate – Santa Elena was abandoned for the larger fortress at St Augustine.
Finally they added a stronger and larger fortress (San Cristóbal) to the northeast, on the Atlantic side of the city.
The largest fortress in the western hemisphere, it took 20,000 enslaved workers 15 years to build, and sits at the summit of a 900m peak.
