Trim Castle is located on the Boyne River at the edge of The Pale–the border between Norman controlled Ireland and Gaelic Ireland. One of the largest castles in Ireland, it was built by Hugh de Lacy and his son Walter in the 12th and 13th centuries.

The first Norman conquest of Ireland began in 1171 with the arrival of King Henry II, determined to rein in the power of Strongbow, who had arranged for himself to be King of Leinster by marrying the current king’s daughter. The Normans very quickly carved out a portion of Ireland for themselves, called ‘The Pale’, which included the area from Dublin to the Boyne River. Hugh de Lacy, as one of these original magnates, was granted the Lordship of Meath and effective rule over much of Norman-controlled Ireland. It continued to be a powerful seat of English rule in Ireland and was in private, English hands until 1993 when it was sold to the nation of Ireland.

Much of the medieval part of the castle is intact, though many additional buildings (and the rebuilding of the old ones) occurred over the centuries. Still visible today are the moat, inner curtain wall, gates, and keep.

Trim Castle is a setting for events in Outpost in Time, a book from the After Cilmeri series.