Real life devolopment - castle/fortress

[Midgardsormr]

Since I doubt you need a truly authentic castle (nobody's going to be hurling boulders at it), you can probably get by with modern framing and a stone or even faux-stone facade. That being the case, the time and expense would be greatly reduced over building a "real" castle.

I know you're just in the speculative stage of this project right now, but you might ask your boss to permit you to make a request for proposals to some exhibition designers. Getting an experienced creative in on the project at the beginning will almost certainly lead to higher-quality brainstorming and a much better end result. Put together a brief on the intentions for the structure—its potential uses, location, general description, and a budget—and ask several designers to pitch ideas to you.

[Elzevir]

Seems interesting !
Quick reply to say that if you look for "castle" or "stronghold" in google images, you'll see pictures of actual castles, but if you try the terms "chateau fort" (the french usage for the same thing), you'll see a lot of sketches from many sources that probably will fit more your searches.
Good luck for your project !