Wallingford Castle was one of the greatest medieval castles in England and the ruins in Castle Gardens are its last surviving visible remains.
Now the town council is weighing up the cost of the repairs, alongside other important projects such as upgrading the Regal Centre, and improving facilities at Bullcroft Park.
Town councillor Adrian Lloyd said: “After the medieval ruins in Castle Gardens were vandalised last year, we had to call out a specialist historic building expert to assess the damage and what had to be done to repair them.
“The ruins are both Grade I listed and a Scheduled Ancient Monument and the town council has a legal duty to maintain them.
“The initial survey revealed that a considerable amount of additional work has to be done to stabilise them, which is why they are currently fenced off.
“A more detailed survey was carried out this year, the purpose of which is to produce a schedule of works – we are expecting to receive this imminently.
“We have been told that the repairs are likely to cost in the region of £0.5m.”
Town councillor Ros Lester said it was suspected that youths had pushed over stonework, causing the damage.
She added: “The ruins were not fenced off and they may not have realised they were damaging the town’s heritage.”
Judy Dewey, curator of Wallingford Museum, said it was vital that the medieval ruins in Castle Gardens were repaired.