Castle Bonuses: redundant or symbiotic?

[Arryn]

Originally posted by General Tacticus:
You better be ready to defend every single one of those towers

As a side note, it's been my experience that forts are attack magnets for the AIs. The AIs seem to prefer to attack forts over attacking nearby, often less well-protected provinces that have magic sites or high income levels (and thus would be a higher priority if it were a human rather than an AI looking at what's juicy to go munch on).

Just a few added words of caution, FWIW.