France
The cathedral in Beauvais is extraordinary. It remains unfinished; having been started in 1227, the work stopped in 1578. Had it been completed on the scale originally proposed it would have been the largest Gothic cathedral in the world.
The chunk which was built measures 72m long and nearly 50m high. The choir alone is 37m long, and when you stand inside gazing upward you grasp the enormous scale of the existing building and sense what might have been.
The builders had terribly bad luck (or weren't very good) because right from the start bits kept falling down, and the 153m high spire collapsed in 1573.
Beauvais centre was destroyed by incendiary bombing when the Germans invaded in 1940 and consequently lacks any old buildings. But the cathedral survived and is well worth visiting. The modern town has attractively laid out streets and squares, with many good places to eat and drink. Fans of French cathedrals could see Beauvais, Amiens and Rouen easily over a couple of days. All three (cathedrals, not fans - there must be more than that surely) are extraordinary and beautiful.
Beauvais is halfway between Abbeville and Paris, off the A16, and south of Amiens.