Sorry if this sounds rather vague. A young relative has developed a passion for castles.
I am seeking a Windows app (preferably, but will accept Linux, or even Mac) which will let one design a castle, preferably drag & drop, and walk through the result (2d is fine, just looking at the floorplan; 3d is a bonus). Bonus if it will critique the design & point out mistakes. Nice if it can the be attacked by an AI enemy to see how well it holds out.
I am looking for an ideal app, but welcome anything at all related to castles. I prefer something serious, but expect that a game might actually be the most realistic.