海军舰队
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 A new, lighter "ladder" boom across Southampton Water was installed in 1907, but within two years this approach had been replaced by a plan to block Southampton Water with a boom made up of floating hulks.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.