At the beginning of Calle del Codo, in the heart of the Austrias district and adjacent to Plaza de la Villa, stands the door of the Torre de los Lujanes (Lujanes Tower). Framed by a horseshoe arch with stone voussoirs that still bear visible quarry marks, this door has traditionally been held to be the oldest in Madrid. Now, a study commissioned by the Real Sociedad Económica Matritense de Amigos del País (RSEMAP) has allowed us to clarify the veracity of this claim.