Just before the town of Corseul where you can find Gallo-Roman remains,you will see a left turn for the Temple of Mars.
On site, the sign was gone so I could not get any information out about it, but once I have found some I will update this post! Feel free to contribute if you know anything about it!
Free to visit, free car park.
Corseul is about 25 minutes from our gite.