The Redoubt Cinema which was due to open in the spring, will now remain closed for 2021.
The intimate movie theatre inside one of the fortress's many casemates, was originally due to open in 2020 — but it's remained closed due to the pandemic.
The cinema was due to show a wide range of cult classics, family favourites and some more recent releases too, running from spring into the autumn.
The Eastbourne borough councillor responsible for tourism, Cllr Margaret Bannister, said:
"We are eager to open this cosy little movie theatre and get the popcorn popping — however it would not be sensible to open at this stage.
"The intimate nature of the theatre and the historical fortress setting does not feasibly allow for one-way systems or social distancing measures that would be economically viable at this time.
"As distancing measures ease, the process of smaller venues returning to normal may take a little more time.
"We want to be confident that when we finally reopen the venue that people will feel safe and will enjoy their experience, returning time and time again.
With so many more activities on offer, particularly our events programme, Bandstand concerts and theatres shows in our bigger auditoriums, this feels like the sensible thing to do."
The fortress will remain closed for 2021, but the borough councils said the neighbouring Pavilion is expected to reopen later this spring, following a full refurbishment, with a large café, bar and outdoor terrace overlooking the Pavilion gardens, serving food and drinks into the evening.