Cádiz is nearly surrounded by water, and the best way to understand its geography is to walk the sea walls. The full circuit runs from the Campo del Sur — the Atlantic-facing promenade below the cathedral — around the tip of the peninsula past the Parque Genovés (a botanical garden squeezed into the northwest corner of the old city) and back along the bay. It takes about two hours at a leisurely pace.
The most dramatic section is the western stretch between the Parque Genovés and the Campo del Sur, where the wall runs directly above the Atlantic and waves hit the rocks below when the swell is up. At certain points there is nothing between you and the open ocean. The walls were built and extended over several centuries to protect the city from the naval attacks that came with extraordinary regularity — British, Dutch, and Ottoman fleets all attempted Cádiz at various points. The walk is best done in the evening when the light is warm and the temperature has dropped.