Fort Marlborough stands on a promontory on the south shore of Maó harbour, one of the most complete examples of early 18th-century British military engineering in Europe. Built from 1710 on the site of an earlier Spanish fortification, it is a star-shaped structure of local limestone with underground galleries, a dry moat, and a sea-facing battery. The fort was designed to prevent enemy ships from entering the harbour; it changed hands between the British, French, and Spanish several times during the wars of the 18th century.
The fort is now a museum, well presented with historical displays and period equipment. The underground galleries are extensive and surprisingly atmospheric. The site also includes Es Castell, the British-planned town built just outside the fort, which retains its original grid plan and several buildings from the occupation period. A short walk from Es Castell connects to Maó along the harbour road.