Carrickfergus Castle has a new roof after a £1m conservation project.
The Great Tower had been leaking at the centuries-old fortification, which is one of the most complete examples of Norman architecture in Britain or Ireland.
John O'Keeffe, the principal inspector of historic monuments, said: "The castle has stood here for over 800 years, it is a landmark and is a part of everybody's history who lives here."
Dr O'Keeffe helped oversee the restoration work in the Co Antrim coastal town.
The new roof was inspired by the construction designs and techniques of the time.
It was formed from slate and lead and built using Irish oak and medieval-period building techniques. Work started last spring.
The roof features 1,000 metres of Irish oak timbers, which laid end-to-end would be as long as 10 football pitches.
Dr O'Keeffe added: "What this has allowed us to do is to place a new roof which is durable.
"It presents a very different appearance of the upper room of the tower, much closer to what it would have looked like in the medieval period."
The tower was built by Anglo-Norman knight John de Courcy in the late 12th century as a focal point of his Earldom of Ulster, and was protected by the sea on three sides. Another knight, Hugh de Lacy, added to the castle complex.
It is the longest continually used castle of its type, from the 1180s until the 1920s, Dr O'Keeffe added.
He said: "Those are figures that are associated with major change in our history in local terms, and in a wider context, in terms of the Anglo-Normans in Ireland, trade and exchange and changes in the markets in Europe and major investment in public buildings of the time, especially churches."
The roof has been replaced a number of times, most recently in the 1930s when a flat roof was installed, which had begun leaking recently.
Minister for Communities Deirdre Hargey said: "This investment has secured the future of this important monument and prevented irreparable damage to the structure.
"Archaeologists and conservation architects within the department worked together to design a historically appropriate roof.
"What has been achieved is an outstanding piece of architecture that safeguards this important and historic place."