A key to Gramercy Park, the tranquil, gated square at the southern end of Lexington Avenue (between 20th & 21st Streets), is one of the most sought-after treasures in all the five boroughs. Who gets a key? Only residents of the beautiful surrounding town houses and apartment buildings. The park, which was developed in the 1830s to resemble a London square, is flanked by two private clubs; members of both also have access to the square. One is the Players Club (16 Gramercy Park South, between Park Avenue South & Irving Place), housed in an 1845 brownstone formerly owned by Edwin Booth (19th-century actor and brother of John Wilkes Booth, Abraham Lincoln’s assassin). Next door at No.15 is the Gothic Revival Samuel Tilden House, which houses the National Arts Club (1-212 475 3424, www.nationalartsclub.org). The busts of famous writers (Shakespeare, Dante) along the façade were chosen to reflect Tilden’s library, which, along with his fortune, helped create the New York Public Library.