Shoreham Fort has been damaged by vandals.
According to Sussex Police Heritage, damage was done to the brickwork of the scheduled ancient monument, which was built in 1857. Images show bricks were dislodged from the fort.
The crime allegedly took place between Sunday and Tuesday.
A spokesman for the force said on Twitter: "If you have any information, please call Sussex Police using the reference 0703 of 24/04/19."