Southwest architecture often incorporates arched adobe walls.  Adobe is a mixture of mud and clay soil with straw, that early Southwestern settlers used as a durable and inexpensive way to build their homes. The thick adobe walls kept the homes cool in the hot summers and warm in the winters.  Interestingly, superstition dictated blue as the paint to go around windows and doors to protect evil spirits from entering.  For others, it symbolized the Virgin Mary's blue mantle, also invoking her protection from evil forces. The soft brown color of the adobe came from the natural color of the soil.

(Note: We intend to build an arched adobe gate in this garden in 2023)
Photo: Southwestern Landscaping, Houszz.com