Águas Livres Aqueduct is a real time commitment — fit it in only if it's a priority.
Allow 45–90 min.
A genuinely awe-inducing piece of engineering — colossal pointed arches that survived the 1755 quake intact, with seasonal walks atop the span over the valley. The catch: the walkable section keeps limited hours, it's west of the centre and easy to miss, and from below it's a quick photo. A treat for engineering and history buffs willing to detour.