older castle walls. The boom was managed from two towers called "dolphins", one just beside the castle and the other on the far side of the water, each with two 12-pounder quick-firing guns of their own, in turn supported by machine guns. The creation of these defences ultimately forced the nearby yacht club to move to the southern end of the spit. 1900–45 In the first years of the 20th century, Calshot Castle's defensive role continued unchanged. The castle's keep was redesigned in 1907 to allow it to house two of the castle's quick-firing guns on its roof. A new, lighter "ladder" boom across Southampton Water was installed in 1907, but within two years this approach had been replaced by a plan to block Southampton Water with a boom made up of floating hulks. A 1910 plan proposed that the castle would be garrisoned in wartime by 10 officers and 154 men, 75 of whom would have to be housed nearby rather than in the fort itself; additional naval personnel would also have been needed to man the support vessels for the boom. A Royal Naval Air Station was built alongside the castle in 1913 to house twelve experimental seaplanes which were intended to support the Royal Naval fleet operating along the Channel. Calshot was particularly suitable for seaplanes, as the surrounding waters and coastline were relatively quiet and calm. The First World War broke out in 1914, when military opinions on the utility of Calshot Castle had shifted considerably. The decision was taken to rely primarily on gun batteries at the two entrances to the Solent, rather than at Calshot and Southampton Water. At the start of conflict the boom was removed and replaced by anti-submarine nets further up the coast' two of Calshot's guns were removed the following year to protect the new nets. The airbase was used for training purposes until 1916, when it took on anti-submarine patrols over the Channel, where the German raids had started to inflict critical damage. Almost 3,500 hours were flown by aircraft from Calshot that year, with over 3,500 being flown in 1917 and over 9,000 in just three months of 1918. Subordinate air stations were created in Bembridge, Newhaven, Polegate and Portland. A cabin was built on top of the keep to oversee air operations, the Castle Yacht Club was taken over for use as the officers' mess and the