We visited Chartes and its cathedral as part of our stay in the Valley de Chevreuse. The cathedral is being cleaned and restored at the moment, a long and painstaking job that was started in 2008. In the modern French way of doing things, it is to be restored to its original 13th century decor, removing the redecorations that were done in the 15th and 19th centuries, and using original materials where possible. This all makes a spectacular difference to the inside appearance: the picture below shows a restored wall on the left and unrestored on the right. The scaffolding is probably a major project in its own right.
The precinct around the cathedral is nice to explore too, but as if to make up for the majesty of the cathedral, the town hall just down the road is quite the ugliest I have seen in France, spectacular in its own way, extraordinary.