I think the most important thing would be to add a dungeon-sized chunk of job exp to Castrum, because anecdotally that's the the reason I saw the most when people stopped doing it. They wanted EXP the most and Castrum didn't give any.

And the droprate for upgrade items should be increased dramatically. The hyper-low droprates just make it seem impossible to get them that way, and you need thousands of coins to buy them all so your only option becomes extreme monotony.

I don't think it's important at all to add mettle to Castrum, because you don't want the rest of the zone to die off and put us back at people afking or instance hopping for Castrum while ignoring everything else.