Two sections of this great wall remain. This probably started as a Gaelic Ringfort on the high point of Dubhlinn (dubh = black in Irish), named for the Black Pool or sheltered harbor. Later the Vikings and later still the Normans enhanced and used this wall as a fortress.
Dubhlin has been inhabited since the Mesolithic age, about 7000 years.
Dubilin City Wall
woolybill1, barbarahirst, mrswoolybill and 28 more