Castell is at the intersection of Farm roads 2768 and 152, on the south bank of the Llano River in extreme western Llano County. Castell was established in 1847 on the north side of the Llano River by German immigrants under the auspices of the Adelsverein. They were led from Fredericksburg by Count Emil von Kriewitz to a site selected by John O. Meusebach to comply with the terms of the Fisher-Miller land grant, which the organization had acquired. The town was named for Count Carl Frederick Castell-Castell, business manager of the Adelsverein. Other Adelsverein settlements—Leiningen, Bettina, and Schoenburg—were established at the same time but did not survive. In 1872 a post office was established on the south side of the Llano River, where the community has since been centered. The site held the first church services in the region, conducted by Rev. Charles A. Grote in 1852. Castell has remained a center of religious activity. The local Methodist and Lutheran churches date their founding from that year. By 1972 the ranching and recreational community had declined to a population of seventy-two, mainly descendants of the original German settlers. A population of seventy-two was still reported in 2000, when the community was the oldest surviving settlement in the county.