In 1623 a survey of the Tudor castle at Southsea found that many of the guns were unusable and that the garrison had no gunpowder stored on site. The deterioration of the castle continued following a fire in 1627 which gutted many of the buildings. It was still in use however during the Civil War and in 1642 it was captured by the parliamentarians. In 1680, following the restoration, Charles II built an enlarged castle with 30 guns.
However by 1770 things had been allowed to deteriorate and a document describes the castle as being a shameful ruin and plans were made for its demolition. However renewed risks of French invasion called for these plans to be put on hold and the castle was further strengthened in 1793 and in 1814.
In the 1820s the lighthouse was added to the castle and this remained in service to 1927.