The existence of the 17th century citadel - a heavily fortified building - was made by accident when an archaeologist stumbled across a "faint annotation" on a map while carrying out research on Derry's cannons and defences.

Dr Brian Scot found a heavily fortified area around the Cathedral on the map which was described as "The Church formerly a Cittidell."

The Deputy Mayor Derry Patricia Logue has welcomed the find and has escribed it as an exciting and significant occasion for the city.

Mark Lusby, Senior Economic Development Officer with Derry City Council, added: "In other European fortified towns with bastions like Derry, the citadel is usually the strongest part of the system, sometimes well inside the outer walls and bastions, but often forming part of the outer wall for the sake of economy. The Citadel was positioned to be the last line of defence should the enemy breach the other components of the fortification system.

"Equally importantly Citadels were most often used to protect a garrison or political power from the inhabitants of the town it is defending. They were designed to ensure loyalty from the town, which they defended. Many commentators in recent years thought that the absence of a Citadel as part of the city's defences indicated the loyalty of its citizens."

It is believed that the citadel was destroyed in the early 1670s after the discovery of a warrant issued by the Lord Lieutenant on 10 August 1667 for the destruction of "the citadel of Londonderry... erected by the late Usurpers so contiguous to the Church there, as that it is a great nuisance to the Church, and perhaps purposely so intended by the erectors."