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You can click on this archaelogical description and an ancient map of Carrickfergus Castle
Carrickfergus is a medieval castle that has, through the ages, been used as a prison, an armory, a military garrison, and during World War II as an air raid shelter.
The Norman conquest of England was an 11th-century invasion and occupation of England led by William the Conqueror. After their successful conquest of England, much of its land was seized by Norman barons. This process began in 1169 when Norman mercenaries gave aid to Diarmait Mac Murchada, an exiled King of Leinster. He solicited help from King Henry II in return for pledging him an oath of allegiance . This venture succeeded, and Henry II then mounted a larger invasion in 1171, resulting grabbing the title and control of a Lordship of Ireland.
At that time, Ireland was divided into a small number of kingdoms whose rulers were ever in the midst of competing for the title of Ard Rí
, the High King over the entire island. There was a lot of jockeying for position and power. Therefore, having a castle as of base of operations was important. Building a castle was doubly important for the Norman lords so they could establish themselves as contenders in the area as well. Normans such as John de Courcy built castles in large numbers to support military and political control in the newly occupied territories.
Since King Henry II did not have the power to do much to support or defend England's lordship over Ireland, English lords could only maintain dominance over an area if they could defend themselves against other power seekers. A castle thus became a necessary fortress to garrison soldiers and give them military capabilities. For the lords, the castle served as a home, a symbol of power, and an indication of high status. In other words, by building Carrickfergus Castle, John de Courcy established an administrative site from which to exert political, military, and economic control in Northern Ireland.
The castle was built by de Courcy after he conquered east Ulster in 1177. He had assembled a small army which defeated the last King Ruaidhrí Mac Duinnshléibhe of Ulaid, and he took the king's land as his own due to its superior positioning. De Courcey chose this place as his home and his headquarters because it was surrounded by the sea on three sides, making it difficult to be taken by sneak attack via the sea. The setting gave a vast sweep of views that also provided useful protection from invasions by land.