John de Courcy was not invited to Ulster. He conquered it. In 1177, this Anglo-Norman knight swept into the east of the province and established himself as a petty king, building Carrickfergus as his headquarters on a basalt promontory that commanded the approaches to Belfast Lough. The inner ward -- a small bailey with a high polygonal curtain wall and east gate -- came first, along with the great hall. The castle's strategic position, originally almost surrounded by sea, controlled both the waterway and the land route into the walled town that grew beneath its walls. De Courcy ruled until 1204, when another Norman adventurer, Hugh de Lacy, ousted him. The castle's loyalty to any particular lord was always negotiable; its loyalty to its own strategic value was absolute.

King John laid siege to Carrickfergus in 1210 and installed English constables. In 1217, the new constable was granted one hundred pounds to build additional curtain walls protecting the landward approaches. The castle served as the Crown's principal stronghold in northern Ireland after the Earldom of Ulster collapsed in 1333. During the Nine Years War of 1595 to 1603, crown forces were supplied through Carrickfergus port. Marshal Schomberg besieged and took the castle in 1689, and King William III first set foot in Ireland here on 14 June 1690. In 1760, a French force under Francois Thurot captured the castle after fierce street fighting, looted it, and sailed away -- only to be caught and destroyed by the Royal Navy.

In 1778, during the American War of Independence, the Scottish-born American naval captain John Paul Jones appeared in the waters off Carrickfergus. His crew was reluctant to approach the castle's guns, so Jones devised a ruse to lure the Royal Navy sloop HMS Drake from its moorings. The two ships fought for an hour in the North Channel, and Jones prevailed -- a small but symbolically significant victory for the fledgling American navy. The castle continued its military life through the Napoleonic Wars, when twenty-two guns defended the east battery (six survive), through the First World War as a garrison and ordnance store, and through the Second World War as an air raid shelter. By 1928, when the army finally departed, the castle had outlasted every conflict thrown at it.

After its transfer to civilian stewardship, many of the castle's post-Norman and Victorian additions were stripped away to restore its original medieval appearance. The banqueting hall was fully reconstructed. Cross-bow loops in the east tower basement, a Romanesque double-window surround in the first-floor chapel, the ribbed vault over the entrance passage, and the murder hole and massive portcullis in the gatehouse all survive as tangible evidence of the castle's medieval architecture. On 29 April 2011, Prince William was created Baron Carrickfergus on his wedding day, reviving a title that had been extinct since Victorian times -- a purely ceremonial honour with no official connection to the castle, but a reminder that the name still carries weight. The castle is a short walk from Carrickfergus railway station, with trains connecting to Belfast. Its walls have weathered 847 years of rain, war, and politics, and show no signs of yielding.