Bishop Maurice de Sully began the current structure in 1163 and construction continued for 170 years.  The result is a Gothic masterpiece and one of the world's greatest buildings.  It's a combination of Romaneque and Gothic designs.  Notable are the Gothic Rose Windows and the spectacular array of flying buttresses.  The buttresses are balanced supports pushing in opposite directions on a central structure which are held up by their combined forces.  Essentially, the Cathedral itself, is supported and works by intentionally playing opposing ends against the middle and against each other.  The magnificient  exterior ornaments, especially the gargoyles around the ledges are unequal.  The gargoyles also serve as rainspouts.  The front of Notre Dame is the point from which all roads of France are measured.