The bastioni — the walls and towers ringing the old town — were rebuilt and reinforced by the Aragonese and Spanish through the 16th century, replacing a smaller medieval circuit. Seven defensive towers survive of the original eleven, and the sea-facing walls have been converted into a continuous pedestrian promenade. Walking the full circuit takes around 45 minutes; doing it at sunset, with the sea on one side and the cathedral towers on the other, is the closest thing Alghero has to a single must-do experience.
The towers are each worth a stop. Torre di San Giovanni (Sant Joan) houses a small archaeological exhibit; Torre dei Cani has occasional contemporary art shows; Torre di Sulis on the southern corner is the largest. The walls connect a series of small terraces with cafes and bars — Caffè Latino is a useful sunset spot if you don't mind paying for the view.