Barrage Vauban is a defensive weir across the Ill, built in the late 17th century to help protect Strasbourg after the city came under French control. From river level it looks broad and severe, but the real reward is the terrace on top, where the view opens over the Ponts Couverts, La Petite France, the cathedral spire, and the old waterways.
It is one of the easiest high-value stops in Strasbourg because it costs little time and gives you the city's geography in one look. Go near sunset if the weather is clear, or use it as a western anchor for a walk through La Petite France before looping back toward the cathedral.