Instead of fighting the crowds in Notre Dame, take the Metro (line 13 - Direction St. Denis-Universite) to St.-Denis Basilique to visit the great basilica of St. Denis.
It is in a poor district, but the church is superb, and not well-known. It is where the Gothic style was invented by the Abbe Suger from 1138 to 1144 (the East end), while the nave and transepts are even more spectacular.
It has some of the most beautiful stained glass in France, some of it at eye level, far easier to appreciate than Chartres, and it was the burial place of the French royal family: 46 kings, 32 queens, and 63 royal children were buried there, including Marie Antoinetee and Louis XVI, and there are many superb effigies, though the bodies were thrown out at the Revolution.
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