How to solve castling effect?

[Taqwus]

Feh. Building lots of castles is rather expensive in gold, gems or blood slaves depending on how you do it -- unless you're astoundingly lucky with fortress sites and fortress events. If somebody's building castles everywhere, that's a lot of gold expended plus time consumed with commander-turns. Then garrisoning all those bloody castles is expensive unless they're undead, most of which defend the gates rather poorly.
You might as well argue that the game encourages having decent armies backed by mages.