I forgot to mention the Great Wall of Paris. They don't really call it that, but I think that name captures the essential idea. One of the kings during the Middle Ages had to leave to go on a crusade, and was afraid that Paris might be attacked during his absence. So, he built a wall around the city. The important thing about this wall is that it's not around the islands - it's actually on the banks. So, it was the first time someone thought of Paris as expanding beyond the river.
The wall that was supposed to protect Paris.