Which Castrum is this?

[MistakeNot]

The Castri throughout Eorzea are temporary constructions built in an extremely short time. Think like one of those flat-packed build-your-own houses. Airlift a few juggernauts of troops and engineers, and a container aircraft full of building parts into the heart of enemy territory, and suddenly you've got a well-defended military base in a strategic location for midnight raids and espionage. It's a key part of the strategy for Garlean expansion, and you can see how they used such techniques in Yanxia as well.

Also consider the camps that the legions of Ancient Rome used. They basically built a fortified camp at the end of each day when marching - built it in a couple of hours.
They also had more permanent camps built along the same design.

The Latin name for such camps was castra (singular castrum.)

Any similarities between the camps of the Roman legions, and the Garlean fortresses are likely not coincidence. The Garlean Empire in FFXIV draws much inspiration from the Roman Empire of our world.