Construction began in 1204 under King John’s command to build a stronghold for defending the city and protecting the royal treasure. What started as a replacement for an earlier earthwork fortification quickly evolved into the administrative heart of English rule in Ireland, serving this purpose from the early thirteenth century until Irish independence in 1922. Access from the medieval town was through a barbican gate and drawbridge positioned halfway along the castle’s northern wall.

The castle represents an early Irish example of a keep-less design, a style that had emerged in France in the early thirteenth century and can be seen in similar fortifications at Limerick and Kilkenny. Its quadrangular plan measures 103 metres long by 55 metres wide internally, with circular mural towers at each corner connected by curtain walls; the southern wall features a distinctive kink in its layout. The corner towers each had their own identity: the Record Tower in the southeast, Bermingham Tower in the southwest, Cork Tower in the northwest, and the Powder Tower in the northeast. A projecting middle tower on the south wall was later replaced by a polygonal tower during eighteenth-century renovations. The entire structure was surrounded by an impressive moat, formed naturally by the Poddle on two sides and by defensive ditches on the remaining sides.

Archaeological excavations from the 1960s through the 1980s have revealed fascinating glimpses of the castle’s medieval foundations. Work in 1961;62 uncovered portions of the original eastern and northern curtain walls along with the northeast corner tower, whilst excavations in 1985;86 exposed sections near the Bermingham Tower, including an arch crossing the moat and the tower’s battered base. These investigations also revealed a large rectangular barbican aligned with the main entrance, complete with a drawbridge pit, and substantial portions of the rebuilt Cork and Powder Towers. Perhaps most intriguingly, traces of a Viking stone-faced bank were discovered at the lowest levels of the Powder Tower, offering a tantalising connection to Dublin’s earlier Norse inhabitants. The curtain walls themselves show varied construction techniques, founded on boulder clay in the upper yard and directly on limestone bedrock where they connected the Cork Tower to the castle gateway.