This is the country’s oldest museum dedicated to architecture and design. Dr. William Thornton, first architect of the Capitol, designed the house in 1801. As its name suggests, the structure is an architectural marvel. Eight sides, though? Nope, try six. Thornton made the house for Colonel John Tayloe III, a Virginia planter, breeder of racehorses, and friend of George Washington, who would come by to inspect the construction site from time to time. Upon its completion (which Washington did not live to see), the Octagon became a favorite social mecca, the Tayloes welcoming John Adams, Thomas Jefferson, James Madison, James Monroe, Daniel Webster, Henry Clay, and their ilk.