It isn't Sur Mer any more, but be careful to tell your satnav it is or you will get directed to the Montreuil in Paris or even Switzerland.
The town is on a hilltop dominating the surrounding countryside. And at some point they have been to a lot of trouble to make their hilltop hard to attack. The walls rise up sheer from the low lying fields.
The ramparts surrounding the town are almost perfect. One breach allows traffic though into the town. There is another access road which enters through a gateway.
And those ramparts are scary. No namby-pamby health and safety fencing. You are 30 metres up. Fall off and you die. I stay as close to the inside edge of the path as I can. I'm not particularly scared of heights you understand. I just prefer not to fall to my death if it's all the same to you.
There is a complex arrangement of ditches and outlying fortresses. I think the idea is that if an invading army breaks through the walls, they find themselves trapped in a ditch with the Montreuillois pouring boiling oil over them.
The town is on the inside of those mediaeval wall. And for the most part it's a higgledy piggledy mediaeval town of small houses and narrow streets. There are a couple of more modern tenements which look like barrack blocks, and certainly out of place. A place to wander.