It was destroyed by Amelia Black and her allies during the events of Act III: Steel. The Ossuary has many fortifications, including 3 Fixed Gun turrets, many special outposts (built directly into its wooden ramparts), many structures (Barracks, Artillery Foundries, etc.), stone and wooden walls, and many groups of units with cannons.
To beat the Ossuary, you have to ally with Havana, effectively protecting yourself from the first huge wave (though on the Hard difficulty Havana may be nearly wiped out stopping this initial attack and need assistance holding off smaller subsequent waves until they rebuild). Make sure you make your base south of Havana (near the area where Amelia initially makes landfall), or this first wave might destroy you. After the first wave, its just small groups of enemies attacking, and they are easily defeated by you and Havana.
To beat the Fixed Guns, use the Monitors that the Americans send and their long-range attack or simply storm the Ossuary with land units, as the fixed guns are incapable of supporting the Ossuary defenders with their land battles (they can only fire out to sea to stop ships). After the guns are dealt with the monitors can switch to assisting land based attacks against the stronghold.
"An ossuary is a place to store the bones of the dead. Sometimes they are boxes and urns, but often an ossuary is a room or a series of connected rooms beneath the floor of a church. As the bones pile up, industrious faithful folk, often monks, decided to take on the gruesome task of using the bones as decorations for the crypts. Fans of scapulas frame skulls and artfully arranged vertebrae. Chandeliers, niches, chairs, heraldic symbols, and more are replicated in bone. The overall effect is chilling and reverant at the same time."