The Porte Saint-Paul, on the opposite side of the old town from the Porte Chaussée, is another fragment of the medieval defences — a single tower with a drawbridge mechanism intact, dating from the 18th century. It overlooks a small park and the surviving section of the Vauban wall.
It's worth a five-minute look on the way to the Citadelle Souterraine, which sits a few hundred metres west along the same line of fortifications.