Finally manage to beat Castrum and so here is my thoughts on Episode 2 of Save the Queen Storyline...
I feel as if we will not be defeating Gabranth this expansion.
This entire battle of Bozja is just one big distraction while Gabranth deals with something else far from this battle zone. The entire purpose of even having this "primal" placed into this Castrum is to help delay any progress to claim the Castrum as they have neither intention nor expectations to keep it under their control. They're just doing what they can to stall for time so the Bozja resistance won't make progress into Bozja and have foothold to finally setup a assault into Garlean territory anytime soon.
Even if we claim the location finally in Map 3, the current story hints that the story for Gabranth may not be finished until 6.X era storyline. If so i am assuming Save the Queen storyline may not be required to access but will still help better understand how X and Y character ended up in the 6.X Gabranth storyline along with extra dialogue options exclusive to players who played and completed 5.X Save the Queen Storyline.