One of the main assets of Spain’s Costa de la Luz are its large stretches of quiet, natural beaches (a far cry from the overdevelopment in many Mediterranean areas).
The Playa de Castilnovo in the province of Cádiz is one of the few pristine beaches that still exist next to a well-established tourist destination. It is in walking distance from Conil de la Frontera, only separated by river Rio Salado (during low tide you can easily cross its mouth by foot, however).
The place has no services, no promenade, and no immediate access by car. Due to its expanse of more than 2km it remains fairly quiet even during high season.