In 1902, the Weymouth Corporation purchased the castle for the town from the Department of Woods and Forests for a total of £150.
During the Second World War the castle probably housed an anti-aircraft battery as part of the defences created around Portland Harbour.
Despite the construction of the Portland breakwaters nearby, the unstable clay cliffs remained vulnerable to erosion; in 1930 the ruins were closed to the public due to safety concerns and the remaining embrasure of the gun platform collapsed in the 1950s.

In 2009, £23,100 was awarded by the Heritage Lottery Fund for an initial survey of the site, which by now had been placed on English Heritage's register of listed buildings at risk of decline, followed by a further £194,700 in 2011 for substantial repair work.
As part of the work, a three-dimensional laser scan of the stonework was undertaken, and a steel and oak walkway was installed around the interior of the castle.
The castle reopened to the public in 2012, and the following year it  was removed from English Heritage's at risk register.
The castle is protected under UK law as a grade II* listed building and as a scheduled ancient monument.
In 1902, the Weymouth Corporation purchased the castle for the town from the Department of Woods and Forests for a total of £150. During the Second World War the castle probably housed an anti-aircraft battery as part of the defences created around Portland Harbour. Despite the construction of the Portland breakwaters nearby, the unstable clay cliffs remained vulnerable to erosion; in 1930 the ruins were closed to the public due to safety concerns and the remaining embrasure of the gun platform collapsed in the 1950s.
In 2009, £23,100 was awarded by the Heritage Lottery Fund for an initial survey of the site, which by now had been placed on English Heritage's register of listed buildings at risk of decline, followed by a further £194,700 in 2011 for substantial repair work. As part of the work, a three-dimensional laser scan of the stonework was undertaken, and a steel and oak walkway was installed around the interior of the castle. The castle reopened to the public in 2012, and the following year it  was removed from English Heritage's at risk register. The castle is protected under UK law as a grade II* listed building and as a scheduled ancient monument.
Sandsfoot Castle