Fort St. Marie-La Roche are constructed in early 1726 in the area now known as Newfoundland and Labrador.

A fort and population center on a relatively unhabituated tundra island with amenities and fortification in place by the time the British arrived? Why on Earth would this not only be abandoned, but entirely forgotten about? Considering where it is and the era this is set, the British Empire wouldn't have just tried to destroy the Fort and sack the surrounding town. They would've greatly appreciated every foot hold they could get. I would suggest the fort is destroyed by anomalous means rather than human intervention to explain why it wasn't just occupied. Could further have the Authority's predecessor, the Auctoritas Imperata, been involved with covering up the destruction.

St. Marie-La Roche remain unknown for years until the Royal Navy discovered the fort in 1935. The Royal Navy were quickly attacked by the fort's cannons and mortars, the ships have to retreat from the site in order to prevent further casualties. The acting captain immediately report it to the Admiralty, in which the Admiralty ordered further inspection and supervising. The Royal Navy continue to watch the fort with their patrol boat until 1940.

The RPC Authority are notified about the St. Marie-La Roche's existences in 1950s.

If the British were the first to reencounter them, then this anomaly would more then likely fall into the sights of Military Intelligence Section XIII¹. Being they're allied with the Authority, that means the Authority would likely find out about it sooner through them.

Also, general grammar fixes all over. I'd run this through some kind of document programs like Microsoft Word or Google Docs.

I do like the base concept of this. A fort of ghost/zombie/ambiguously immortal French guard still loyal to the king and still holding onto the relations of their old empire (ie, hating the British/Spanish and being buddy buddy with the Americans next door). There's also a kind of hole when it comes to the French in that period of the lore. Could be a good avenue to expand on things through interviews between implanted agents and soldiers from the fort.

Personally, I think it'd be more interesting if the Fort was a ye olde anomaly containment site that suffered a catastrophic breach and as a result all the people stationed there were made immortal and shackled to the location for eternity. Steadfast in their duty, they attack anyone that approaches to stop the further exacerbation of the breach.

General Idea/Elevator Pitch
An 18th century French fortress/settlement named "Fort St. Marie-La Roche" that are isolated for hundreds of years and hostile towards everything within their radius.

Despites being outdated, their weapons (musket and pistol) are capable to penetrate modern military-grade armory. The human population of the fortress remain static, with its people are generally immortal and able to regenerate their wounds. Most of the population still shows loyalty towards the French monarch, even after the dramatic fall of the monarchy in France.

Central Narrative
Fort St. Marie-La Roche are constructed in early 1726 in the area now known as Newfoundland and Labrador. For several years, St. Marie-La Roche operates like a normal French Fortress and able to grow it's own town without any anomalous activity inside it.

The fortress are raided by British mortar in the Seven Years' War between French and British. After the raid, some personnel and population escaped the fort because.. well.. the forts are destroyed, but some choose to stay in the fort, including the Commanding Officer, Colonel Jean-Paul de Natalie. The fort lost contact with the French Navy and presumed destroyed by the government, thus forcing the commands in St. Marie-La Roche to act on their own.

St. Marie-La Roche remain unknown for years until the Royal Navy discovered the fort in 1935. The Royal Navy were quickly attacked by the fort's cannons and mortars, the ships have to retreat from the site in order to prevent further casualties. The acting captain immediately report it to the Admiralty, in which the Admiralty ordered further inspection and supervising. The Royal Navy continue to watch the fort with their patrol boat until 1940.

The RPC Authority are notified about the St. Marie-La Roche's existences in 1950s. The researchers are aware that St. Marie-La Roche are hostile towards everyone, therefore they're directed to keep safe distances from St. Marie's detection and attacking radius.

The first infiltration attempt were conducted in 1958, which resulted in two operatives being killed by the fort's personnel. After countless attempt and evaluation made by the Authority, they've decided to keep distances from the fort, considering the attempt of infiltrating and/or destroying is deemed
"not very worthy" and violates the Authority's value.

Why Shall I Care About This? AKA The Hook
I believe that from this small concept of isolated hostile fortress are able to be expanded into a far more wide story and able to be integrated into broader RPC Universe.

Notes
Rain me with your crits, this thing needs more improvement. Contact me in Discord (Coordilatte#9929) if you need to.