The former draw-well of the castle, 37 feet deep, was situated in the keep. It was famous for medicinal qualities. The lower portion of the building is now used as a magazine. The walls of the tower are 9 feet thick, and the corner stones, or quoins, are of yellowish limestone, which was probably quarried in the County Down on the opposite shore of the Lough.
The building of the castle is generally ascribed to John de Courcy, and, although there is no direct proof that this was the case, many facts tend to support the supposition. In the first place, as De Courcy settled a colony in Carrickfergus shortly after his conquest of Ulster, it is most likely that he would provide some means for its protection. Again, it was for a long time the hereditary property of the Earls of Ulster, who were descended from De Courcy. The ancient seal of the mayor of the town bears a spread-eagle, which was the De Courcy crest, and several coins of Henry II.’s time have been found near the building. In a preface to State Papers the editors say that “the oldest fort in Ulster is Carrickfergus, built in the days of De Courcy, and never out of the possession of the English.”
In 1605, the Lord Deputy applying for means to have it restored, remarks it was “founded by his Majesty’s ancestors, and much needing repair.”
It is likely King John stayed in the castle during his visit to Carrickfergus in 1210, and an order is preserved to the Bishop of Norwich to buy supplies for it that year. It appears to have passed into the hands of Hugh de Lacy when King John granted him Ulster, but in 1223 a garrison was to be placed in the castle lest it should be attacked by De Lacy, who was then plotting against the King.
The following year a band of knights and soldiers were despatched by the Earl of Pembroke for its further defence. Although it was being besieged by Hugh they managed to get into the fortress safely, and the siege was then raised.
Two years afterwards the custody of the castle was granted to Hugh de Lacy’s brother Walter.
In 1245 an order was issued for its repair, and later (1253) it was assigned by the King as part of the dowry of Eleanor, Queen Consort.
In 1315 the castle was besieged by Edward Bruce, and Lord Mandeville, who endeavoured to relieve it, failed to do so. The gallant little garrison held out for more than a year, and it is said they were reduced to eating eight Scotch prisoners who had died within the walls. Upon the arrival of King Robert Bruce to aid his brother, the fortress was surrendered. After the death of Edward Bruce the castle passed again into the hands of the English, and it appears to have been the only place in Antrim not in the possession of the O’Neills after the assassination of the Earl of Ulster, 1333.