The closest is Marinid Walls of Algeciras, 1 km north-east of the city centre; the most famous is Moorish Castle (#771 of 9,035 worldwide) 10 km east. The oldest dated site in the circle is Castle of Jimena de la Frontera, founded c. 750.
The 100 km circle reaches into 3 countries: Spain (35), Gibraltar (2), Morocco (2).