Today we visited Chartres which is about 90km (55 miles) outside Paris or one hour by train to see the famous cathedral there.
I find medieval cathedrals quite fascinating since they provide a window into not only the religious life of the time but also the political, economic and social systems. It always amazes me how much could be achieved in stone, and what people living often on the brink of starvation would sacrifice to build churches that would generally not be finished in their lifetime (or even their children's or grandchildren's).
At the same time they provide evidence for the limits of medieval scientific and engineering knowledge and show how slowly knowledge grew before the industrial revolution.
Take for example the towers on the cathedral. The south tower (on the right) was built along with most of the rest of the cathedral in the early 13th century whilst the north tower (on the left) was built 300 years later in the 16th century, and while the style is quite different the engineering limits are fundamentally the same. Yet another 300 years later Eiffel was able to build a tower of steel nearly three times as tall as a temporary structure.