Make an appointment to discover American architecture and design.. In Short Anyone wandering about Washington Square has seen the stately structure on Sixth St. The Italianate mansion, built in 1845, is decorated in American fine and decorative arts from the first half of the 19th century. If you want to learn about the city's built history, there's really no better way. The centerpiece of the Athenaeum's collection is the Philadelphia Architects and Buildings database--165,000 buildings, architects, images and projects culled from about 30 area repositories. An online version of the database lets you search for specific projects.