As I recall Venice was also built in a marsh for defensive reasons and had no walls.
Of course when the marsh disappears, so does your defense.
Venice is still around, but prudently joined the rest of crazy Italy. ;^) Marshes surrounded the hilltop where Alfred the Great holed up to arm and build his army. It was a great defensive position, but as the saying goes, the best defense is a good offense. The marshes weren't going to protect him forever.