It's the seawall that was built by the Royal forces, led by Richelieu, to keep seaborne reinforcements from either supplying the city or breaking the siege.
You can see the sea wall at the lower part of this painting.
Here's a bird's eye view diagram of it.
http://en.wikipedia.org/wiki/Siege_of_La_Rochelle#mediaviewer/File:Pompeo_Targone_seawall.jpg
