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Ireland's oldest complete building and the first to use mortar, 13th-century Reginald's Tower, the city's key fortification, is an outstanding example of medieval architecture. Its 3m- to 4m-thick walls were built on the site of a Viking wooden tower. Over the years the building served as an arsenal, a prison and a mint; it now houses a museum recording the city's Viking and early medieval history. The sparse exhibits include the tiny but exquisite Waterford Kite Brooch, made around 1100.