Fort Boyard is a Napoleonic fort located on the west coast of France. Building started in 1801 and was completed in 1857.
The filming location was this 116 year old stone fort surrounded by water.
The cast and crew had to ferry every morning by boat to the fort for filming.
Some days the sea conditions were very rough so cast and crew had to deal with "Mal D'Mare" in addition to their normal work.