I have not heard word if customizable castles are back in the game.
I do remember in the original M:TW you could build add-ons to the castle to improve its defensibility. Such as Canon towers or special walls that Jutted out of the castle so they could hit targets far away etc.
This is somthing that was sorely missed in R:TW. Apart from the Usual options of upgrading settlements to towns to Cities etc. you could not for instance customise the city mith defensibly moats and stuff.( Yes I know moats are mainly from medival times but you get the idea)
You could even take this a step further so you can build stronger doors or better gate defenses to make your castle stronger at keypoints you find are your weakness on the battlefield.