What am I missing?

[Korbei]

Once you get to Rank 15 and do Castrum, it does seem completely pointless. Weapon grind is much more efficient outside and as much as I enjoy FATE grinding, it's nothing new or special. The only reason I have to go in is the chance to lose mettle.

[MsMisato]

You are honestly not missing much. Bozja is a warzone with super hyper aggressive mobs that will relentless chase you to the end of the earth. The camp isn't a safe zone. The story and map is gated behind the ranking system. I mean you can hit castrum at rank 10/11 and that's that. THe only thing castrum is good for afterwards is coins for armor and i believe a very small chance of an armor piece from a chest. The hardest part and the one part I didn't like is finding an instance with castrum getting ready to pop and a group that will work together to complete it. But if you miss it you have to wait an hour for it to repop.