There have been two incidents of damage being caused at Shoreham Fort in May, one during an attempted burglary, following an act of vandalism last month.
Police received a report of an attempted burglary at Shoreham Fort sometime between 9pm on Monday, May 15, and 9.30pm on Tuesday, May 16.
Damage was caused to a steel door, police said.
The paintwork had been scratched and chipped and the door had been dented.
The cost of replacement is around £800, police said.
A further report to the police was made after graphitti written in marker pen was discovered at the fort.
This is believed to have happened sometime between 8pm on Sunday, May 21, and 1pm on Tuesday, May 23.
The damage follows an incident in April in which an information board was ripped off the wall and thrown down an 18 foot drop, according to a spokesperson.
Original brickwork and an air vent were also damaged, added the spokesperson.
Any witnesses or anyone with information about the incidents are asked to report online to https://sussex.police.uk/contact-us/response-to-appeal/ or call 101 quoting serials 1668 of 16/05 and 738 of 23/05.