Remains of a medieval castle believed to be one of the first built in Ulster have been uncovered in a former County Tyrone British military base.
They are thought to date back to the early 14th Century and to have been built by Irish chieftain Domnall O'Neill.
Archaeologists have examined the site in the centre of Dungannon and recovered thousands of artefacts, but it has now disappeared under tonnes of concrete in preparation for development.
While the sealing of the site will preserve the remains, it prevents further exploration.
Dr Tom McNeill, of the medieval history department at Queen's University Belfast, said today a chance to fully look at an important part of Northern Ireland's past was being missed.
Because of the site's role as a security base for decades no one had been able to gain access.
Now after a brief glimpse it was unlikely it would be properly excavated again for another generation, he said.
Dr McNeill, who visited the site during the excavations, added: "It is in many ways an opportunity seriously missed to have a look properly at what is there."
The site at Castle Hill is currently being vacated by the Police Service of Northern Ireland, which had accommodation barracks there.
They took it over in the late 1980s when soldiers who had been based there since the start of the 'Troubles' in 1969 moved out.
The local council is understood to be negotiating to take over the site , owned by the Orange Order, and turn it into a town park.