Premise: 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.

Hypothesis: Puerta del Sol is known for its lack of congestion. 
Relation: contradiction