My English campaign that ended in world domination had a policy of 3 castles, but that distorted later on. I had Bern, Staufen and Innsbruck as my 3 when I was fighting against Italian and Eastern Eurpean factions - but later on they were too far away so I just kept Innsbruck. Due to the Timurid threat I made castles in Mosul ane either Edessa or Damascus (can't remember). These provided me with enough Retinue Longbowmen to defend the bridge over the river to the west of Baghdad.

In the Americas, Caribbean (Habana) looked like a good place to base a castle as it had no resources to trade. In the end I didn't need it, having sent over 4000+ mercenaries and generals to defeat the Atzecs, recruiting more local mercenaries.

Call me boring but I've started another English campaign, this time as a short game and focussing more on diplomacy etc. I've kept my castles in Nottingham, Caernarfon and Caen, as there are Scots to the North, loads to the South, and I haven't bothered to change Caernarfon just yet. I also want a woodsman's guild in Nottingham.