1. Castrum doesn't offer mettle, so your better off farming mettle outside the castrum unless you need it for the questline.
  2. Castrum offers augmentations, but they are a low drop chance and are only for giving you haste (in bozja) and no other upgrade to your gear.
  3. Castrum offers coins for gear and augments, but the small amount of coins you get are better off spent on getting regular gear pieces for each set (Aiming/Striking/Fending/etc) unless your really serious about saving the 500 or 900 needed for augments.
  4. Castrum offers fragments, but most of the time for people new to it, you got unusuable items in your inventory until you raise your rank after the first go. And to raise your rank you need more mettle which means avoiding Castrum till your rank is higher.
  5. Castrum offers fun fights, but if you don't know the mechanics and/or don't use an essence, your gonna be lying on the floor for a good portion of the fight. And each death costs you mettle.
  6. Castrum offers 5 non-fragment non-coin rewards. Which are a minion, field notes, and 3 orchestrion rolls. But once you get those (or buy them) there is less incentive to keep doing the castrum.
  7. The field notes especially, as that is akin to only 1 lockbox on subsequent times you get it which is half the reward for successfully completing a normal Critical Engagement but for more time instead. Which the 'Kill It with Fire', is one of the easiest CEs of Bozja has the potential to offer mettle, 2 lockboxes, and field notes which can be turned into a 3rd lock box for a fraction of the time.

So lets say you don't care about augments. What is the most promising reason to do Castrum after the minion, the regular bozja gear (from coins), the field notes, and the three orchestrion rolls? A title called Siege Liege. But that title comes after running it 50 times.

And to get to the point to do the Castrum requires sinking a fairly good chunk of time and effort into reaching rank 10 if you are starting fresh.

Maybe if the castrum offered mettle and lockboxes as a CE there would be more of an incentive to bother doing it. As Otherwise it seems like a bad time sink.