The Solent provides access to the major harbours of Portsmouth and Southampton. Given the naval sensitivities of the area this seaway was heavily defended by a series of fortifications. The western entrance was defended by Hurst Castle built on a spit of land projecting into the Solent. The earliest phase was a Tudor fort contemporary with Deal and Walmer Castles in Kent. The castle was adapted in the Napoleonic Wars, and expanded in the 1850s, 1860s and 1870s. The English Heritage guidebook is by J.G. Coad.
Yarmouth Castle is located opposite Hurst Castle on the Isle of Wight. This was also a Tudor foundation. The guidebook is by S.E. Rigold.
Calshot Castle is located at the mouth of Southampton Water. This was also part of the Tudor defences of the Solent. During the First World War, Calshot became a seaplane base to protect against submarines. The guidebook is by J.G. Coad.