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.