Revolutionary and Napoleonic Wars at the turn of the 18th and 19th centuries led to some of the castles being re-garrisoned and improved, as part of the development of a range of gun batteries around key locations. Some of the fortifications such as Sandgate, Southsea, Hurst and Pendennis, protected strategic locations and these were extensively modernised. Hurst, for example, was redesigned with batteries of heavier 36-pounder (16.3 kg) weapons, and Pendennis was equipped with up to 48 guns. Sandgate's keep was rebuilt to form a Martello tower as part of a wider programme of work along the south coast. New gun batteries were constructed at Deal, Sandown in Kent and Tilbury, while Fort Mersea was brought back into service complete with a new battery as well. Calshot Castle was renovated, Southsea's defences were extensively modernised, as were those in Hull, with the castle and the south blockhouse being refitted. Some of the Device Forts worked in conjunction with the volunteer units raised during the wars to counter the threat of a French invasion. Walmer Castle was used by its captain William Pitt the Younger – then both prime minister and Lord Warden of the Cinque Ports – as the base for volunteer cavalry unit and a fleet of 35 armed fishing boats called luggers. Nearby Deal also had units of infantry and cavalry, called fencibles and in 1802 units of bombardiers recruited by Pitt carried out military exercises at the castle. Calshot was used to store munitions for nearby Sea Fencibles. Pendennis held a new volunteer artillery unit, which was used to train other garrisons across Cornwall. The government coastguard used some of the fortifications as bases to combat smuggling. Calshot was a good location for interception vessels to lie in wait and, by the middle of the century, two officers and forty-two men were stationed there; Sandown Castle in Kent was also used by the coastguard for anti-smuggling operations. In the coming decades some forts were declared obsolete and put to new uses; Portland was disarmed after the war and converted into a private house. Gravesend was superseded by the New Tavern Fort and demolished in 1844. Meanwhile, the ruins of Netley Castle were transformed into a Gothic-styled house from 1826 onwards. 1850–99 From the mid-19th century onwards, changes in military technology repeatedly challenged the value and composition of Britain's coastal defences. The introduction of shell guns and steam