Remains of a 410-year-old fort have been discovered on the banks of the River Foyle.
The bastion fortification was built by Sir Henry Docwra in 1600 at Dunnalong, near Strabane in Co Tyrone, as part of his efforts to gain control over the Irish chieftains.
University of Ulster researchers used aerial laser technology to reveal the Plantation-era settlement, which even included a brewhouse to provide beer to the settlement downstream at Derry.
UU researcher Rory McNeary said: "This was a substantial fortification in its day comprising of a five-sided enclosure with four bastions, all the buildings associated with a sizeable garrison, the re-fortified and ditched strong-house formerly belonging to the O'Neills."
He said there was a market-place as well as brew-house. Today's researchers have to analyse the remaining earthwork, which is still buried deep beneath the ground.
The team used airborne Light Detection and Ranging (LiDAR) data obtained from the Rivers Agency as part of the research for the Northern Ireland Environment Agency. The UU is testing its potential use for archaeological mapping in coastal and freshwater areas.
Mr McNeary added: "This is a real coup and the technology will allow archaeologists to pinpoint future geophysical surveys and excavations."
Docwra was sent by Lord Mountjoy to occupy and fortify the town with a large force of men. His main task was to keep in check the Ulster chieftains and try to get them to cooperate with the Crown.
He landed virtually unopposed at Culmore on 16 May and, after fortifying the existing castle there, marched to Derry several days later.
Eventually the area became part of the estate of the Earl of Abercorn.