Refurbished Delapre Abbey will open on time despite flytipping and damage after a group of travellers stopped at the site.
The Guildhall has said the clean-up will cost £5000.
The damage was found after a group of travellers were moved on from the parkland, following intervention from the Countywide Traveller Unit.
Despite the damage to the park, the historic attraction is still due to open to the public for the first time in its 900 year history this summer, following a £7 million investment from Northampton Borough Council and the Heritage Lottery.
Councillor Phil Larratt, deputy leader of Northampton Borough Council, said:
“We are incredibly proud of the work we’ve done at Delapré Abbey. We have spent several years lovingly restoring the property to create a new tourist attraction, safeguarding the future of this marvellous piece of our heritage, which we know the local community are really proud of.
“Fly-tipping and damage like this is unacceptable in any area of our town, but it is particularly galling to see a green flag park defaced in this way. We will not tolerate this behaviour and we will continue to take action against anyone causing damage anywhere in the town.
“We will not be derailed by this set-back and are working hard so that people can start to enjoy the restored Delapré Abbey this year. In light of this incident though, we are considering whether further steps are required to secure the site.”