Questions regarding my strategies

[Sombre]

I think the defense value is irrelevant - your enemy won't sit there sieging your second castle, they'll continue to raid your provinces or go right for the cap. I'm really not sure paying attention to the admin is worth it either - even the worst fort still unlocks the resources in the province itself and being cheaper and faster to build than those with admin, you can simply get more of them. More forts means more mages and even with the admin disparity, probably more troops overall too.