The heart of the city is Viejo Madrid, Old Madrid, a largely 16th-century city built upon the layout of narrow, winding streets of an old Moorish settlement.
Old Madrid was mostly constructed during the 16th century. 
mnli hypothesis: Old Madrid was mostly constructed during the 16th century.  premise: The heart of the city is Viejo Madrid, Old Madrid, a largely 16th-century city built upon the layout of narrow, winding streets of an old Moorish settlement.