NUMBER 10 Downing Street, at that time the most famous address in the world, is one of three gracious seventeenth-century houses built by George Downing, a Harvard man who returned to the country of his birth, became a Cromwellian civil servant, and designed No. 10, No. 11, and No. 12 as "large and well-built houses, fit for persons of honour and quality, each house to have a pleasant prospect into St. James' Park."