The Federal Building was built up to the sidewalk property line because its predecessor of 1901 remained on the east side of the block (it was demolished in 1964). Castle collaborated with San Antonio architect Willis on the design. A limestone-clad raised basement forms the building’s base, with three stories in buff brick above, organized in eight bays defined by pilasters with aluminum-framed windows and geometrically patterned aluminum spandrels. The building finishes with a simple, flat cornice. Entrances framed in limestone portals are in the end bays. The simplified orders and low relief ornament are typical of the stripped classical mode for federal buildings in the 1930s. Two-story wings added in 1964 use a matching brick and similar detailing, but lacking a cornice, they have an unfinished appearance.