The walls of Óbidos run unbroken around the entire old town for about 1.5 km, and you can walk the whole circuit in 30–40 minutes. The path is uneven, narrow, and has no railing or rope on the inside drop — children and the unsteady should be careful. The reward is the best possible sense of the town and the views over the surrounding plain and aqueduct.
Access is free and via several stairways from inside the town. Go in the late afternoon for the soft light on the whitewashed houses below.