Castle Bonuses: redundant or symbiotic?

[General Tacticus]

Finally, stupid as it may sound, I dream of a defensive realm, each province with it's own tower, as far as the eye could see. Would this be stupid, or prudent?

You better be ready to defend every single one of those towers, because if you don't, you'll find that the nasty neighbours suddenly has a base to build troops right in your territory, instead of having to ferry them over 4 or 5 provinces...
Better stick with a few strategically placed and well defended fortified cities, IMHO