The antonyms for the phrase "castle buildings" would be those structures that are not fortified, grand, or prestigious. These could include modest houses, simple cottages, or small cabins. Informal or rustic dwellings, such as tents or trailers, would also be antonyms. Building designs that are modern, minimalist, and sleek would contrast with the intricate architecture of a castle. Additionally, structures that serve utilitarian purposes, such as warehouses, factories, or parking garages, would not share the grandeur of a castle. Antonyms of castle buildings are likely to differ significantly in terms of size, material, design, and purpose.