The antonyms for castle-buildings are vast and diverse. The first that comes to mind is perhaps the lack of any building at all, an empty plain or field. Alternatively, one could interpret the antonyms as smaller, less impressive structures such as small homes or huts. Another antonym might be modern, industrial buildings like factories, power plants, or skyscrapers. Furthermore, one could consider building such as those built for function rather than style, such as storage facilities or army barracks. Ultimately, the antonyms for castle-building encompass a wide range of options from sparse, unadorned landscapes to functional, albeit unremarkable, structures.