The closest is Palace of Viceroy Laserna, 3 km south-west of the city centre; the most famous is Reales Alcázares — #68 of 8,133 worldwide — 77 km north. The oldest dated site in the circle is Roman Theatre of Cádiz, founded c. 50 BC.
The 100 km circle reaches into 2 countries — Spain (47), Gibraltar (2).