Donjon de Rouen 
Description
The only surviving part of the city's castle. Today the dungeon tower is home to an escape game, but you can visit it during the mid-afternoon most days to have a look around (though I'd missed those times by the time I got there).
Charge
Free
Getting There
Latitude: 49.446422; Longitude: 1.094363
What3Words: speaking.slipped.homing
