I personally think the best way to handle this is to remove Mettle loss with Castrum so people are not so afraid to die in it and take part in it. That or reward enough Mettle to be worth the risk of Mettle loss or can cover the Mettle lost.
The most negative reason many feel they don't want to farm Castrum again until they are Rank 15 or only want to do it once for story only is because of the large amount of Mettle lost just trying to beat it.
Easy to avoid mettle loss if they know what to do and have a good organized team but that takes time to prepare and luck for having random R10 players put into the same Instance you're in that wants to do castrum and if they know how to do castrum.
If anything once Rank 10, staying in Bozja to do random things until Castrum is near spawning is more of a long Queue time for Castrum to pop and another shorter queue time to see if you can be among the 48 to take part in castrum. I have been in a few instances where we manage to get all 48 max players into castrum but a good 10 or more players who want to do castrum did not get in because of the 48 player limit.
As for map design, it reminds me of Guild Wars 2 Icebrood Saga Drizzlewood Coast Map a bit but not so open and a bit restrictive. Players progress through the map as a community within the same instance to eventually reach the Fortress where the final boss is and within that Fortress are light raid level bosses and mechanics designed to use everything they got in Drizzlewood Coast (from their mastery and items) and learned through the course of the game to beat those bosses from a Giant Fortress Tank that requires players to defend players controlling their own tanks, throwing explosive grenades, and using sieges to break its armor so the players can attack the Fortress tank directly to eventually the large scale Dragon Champion boss at the end of the fortress as the enemy's last defense against all the players that help fight through the fortress.