Going back to the OP topic, my take:
The benefits/pros from castling:
-Protection of magic sites, temples, labs & recruitment of indep exhotic mages/troops.
-Increased income (depending on the fort's admin).
-Allows recruitment of national troops & commanders.
-Increased supply & resources (depending on admin).
-Protects troops & commanders from surprise attacks.
-The province cannot be lost in one turn to raiders.
The cons:
-Cost in gold!! while castling your valuable provinces can be beneficial, these benefits are less & less clear the less valuable the province is, as the opportunity costs of investing that gold elsewhere are higher.
-Extreme castling requires a low cost fort to be viable (300 gold ones), and these either have low admin ( Mausoleum & Watch tower, which have the drawback of a serious penalty in early income & resources) or high selection costs (Wizard tower at 120 design points).
-A castled province prevents the owner from immediate retaliation vs raiders via movement/distant summon spells: the defender teleporting mages & armies will show up inside the walls, leaving the raiders a full turn to do whatever they wish. This makes the fully fortified empire very vulnerable to burnt land tactics & multiple strikes:
Strike at 5 or so forts at the same time, does your enemy have powerful enough armies/fliers in range to retaliate (or inside the sieged fort)? if yes tax to 200% & consider moving on, if not consider storming in or pillaging. Do not forget that if you storm the fort you might run into the relief force that just teleported in, so be ready to take some losses.
Only distant attack spells with no allegiance can hit the invaders doing this before they act, but these spells are pretty weak in the early & mid game.
-The defender must be ready to fight & retaliate on multiple fronts, otherwise every single fort lost to a multiple strike will become a stronghold for the enemy in the middle of his lands. The defender must also be able to handle the attacker's main force with his own, otherwise he will lose a fort every 2 turns.
-Domes are counter-productive with this strategy as they prevent the defender from reinforcing magically, so research those army bLasting & province bLasting spells & cast away.
Chew on the above & make your turtling opponent suffer.
