Bastion 1 Hilsea Lines

The Hilsea Lines are a line of 18th- and 19th-century fortifications built at Hilsea to protect the northern approach to Portsea Island, an island off the southern coast of England which forms the majority of the city of Portsmouth and its key naval base. They are now used as a greenspace and leisure area, also known locally as Foxes Forest.

Source: Wikipedia