Southsea Castle, built in a few months in 1544 by order of Henry VIII, was part of a vast coastal defense plan aimed at protecting the Kingdom of England from potential invasions by the French and the Holy Roman Empire. Constructed following the Italian model of fortification known as the "trace italienne," the castle features a square central keep, angled bastions, and artillery platforms to the east and west, representing an evolution in military defense of the time.

A few months after its completion, King Henry VIII witnessed the tragic sinking of his flagship, the Mary Rose, during the Battle of the Solent in 1545. In the centuries that followed, the castle was involved in the English Civil War, when it was captured by parliamentary troops in 1642. Over time, the fortress was renovated — with additions such as a 19th-century defensive tunnel, additional batteries, and even transformed into a military prison in the Victorian era — until its final military decommissioning in 1960 and opening to the public as a historic attraction.