The castles are watch towers outside the city walls. The castle is Dublin Castle and is repeated three times because of the mystical significance of the number three. The castles are not castles but represent three gates into the ancient Viking city.

While the term "Dublin Castle" usually refers to the major historic seat of British and then Irish government in the city center, Dublin technically has several fortifications because it was the strategic heart of the "Pale" (the English-controlled area of Ireland). The most famous is the primary Dublin Castle, which dates back to 1204 and served as a fortress and royal palace. Another is Drimnagh Castle, the only castle in Ireland that still has a water-filled moat, which was a vital defensive structure for the city's outskirts. The third often cited is Ashtown Castle, a fortified tower house located within Phoenix Park that dates to the 15th century. These multiple "castles" reflect Dublin's long history as a contested territory where various noble families and the ruling English administration needed fortified residences to protect against raids and rebellions. Today, they serve as museums and heritage sites, providing a fascinating timeline of Ireland's architectural and political evolution over the last 800 years.