The latest issue of Burgs & Bailiffs just came out - a lot of great stuff in there, including an article by yours truly!
Based on my historical research, I generalized the cost of building a castle into a generic format usable in any RPG system.
You can find the latest Burgs & Bailiffs HERE.
I also wrote a handy-dandy spreadsheet calculator to figure the costs out for you - you can find that HERE.
Stay tuned for some follow-up posts on castles in the coming weeks, including some case studies.