One of the busiest places in Madrid, Puerta del Sol (Gate to the Sun) is not only the heart of the city but the starting point for all Spanish motorways, also knows as km0. It began as one of the gates that surrounded a 15th-century fortressed city, and is now a popular destination for locals and tourists for shopping, dining, recreation or just good people watching.  Of note is the Post Office and its clock tower, built in the 19th century and the plaza's oldest building. If you happen to be visiting over the winter holidays, thousands gather here to bring in the new year.