From an old gamerescape interview:

F: There’s a silhouette visible from Monument Tower in Coerthas that looks exactly like Castrum Marinum, but the size and angle seem to be all wrong. Does Marinum have a twin out there?

KF: Oda-san says that this is a place called Castrum Aquilonis. The Empire started building it, but it was way too cold; the climate was way too harsh. They stopped halfway through, it’s now abandoned. That’s all I can say, because that’s all that’s been written.
There's also a small map of every castrum and Garlean installment in Eorzea in the Tales from the Calamity section on page 54, but I didn't see any additional information on Aquilonis on a quick scan.