A peaceful walk takes you around the inland part of the old town's perimeter between two lines of city walls. The inner walls are mainly Roman and date back to the 3rd century BCE, while the outer ones were put up by the British in 1709 during the War of the Spanish Succession. The earliest stretches are a mighty 4m thick. There's a small but helpful interpretation centre (Catalan, Spanish and English).
Entry is from the Portal del Roser, where Avinguda de Catalunya touches the west side of the old town.