The 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.