Carrickfergus Castle (from the Irish Carraig Ḟergus, meaning "cairn of Fergus" the name "Fergus" meaning "strong man") is a Norman Irish castle in Northern Ireland, situated in the town of Carrickfergus in County Antrim, on the northern shore of Belfast Lough. Besieged in turn by the Scottish, Irish, English and French, the castle played an important military role until 1928 and remains one of the best preserved medieval structures in Northern Ireland. It was strategically useful, with 3/4 of the castle perimeter surrounded by water (although in modern times only 1/3 is surrounded by water due to land reclamation). Today it is maintained by the Northern Ireland Environment Agency as a state care historic monument, at grid ref: J4143 8725.
Carrickfergus was built by John long man de Courcy in 1177 as his headquarters, after he conquered eastern Ulster in 1177 and ruled as a petty king until 1204, when he was ousted by another Norman adventurer, Hugh de Lacy. Initially de Courcy built the inner ward, a small bailey at the end of the promontory with a high polygonal curtain wall and east gate. It had several buildings, including the great hall. From its strategic position on a rocky promontory, originally almost surrounded by sea, the castle commanded Carrickfergus Bay (later known as Belfast Lough), and the land approaches into the walled town that developed beneath its shadows.
It appears first in the official English records in 1210 when King John laid siege to it and took control of what was then Ulster's premier strategic garrison. Following its capture, constables were appointed to command the castle and the surrounding area. In 1217 the new constable, De Serlane, was assigned one hundred pounds to build a new curtain wall so that the approach along the rock could be protected, as well as the eastern approaches over the sand exposed at low tide. The middle-ward curtain wall was later reduced to ground level in the eighteenth century, save along the seaward side, where it survives with a postern gate and the east tower, notable for a fine array of cross-bow loops at basement level.