Little remains of medieval Madrid , but there are glimpses of the oldest parts of the city in the Arab walls and several attractive small plazas , as well as many other discoveries to be made in this area that extends from the River Manzanares and Palacio Real ( Royal Palace , actually the legacy of the later Bourbon rulers ) to the lively , congested Puerta del Sol , still the nerve center of Madrid .
Puerta del Sol is known for its lack of congestion .