Trim castle standing on the banks of the river Boyne, is reputed to be the largest Norman castle in Ireland. Hugh de Lacy erected a motte here in 1172 possibly on the site may of an earlier Irish fort. In 1174 the castle was attacked and destroyed by Roderick of Connaught. Hugh de Lacy rebuilt and improved the central tower and keep it was not completed until the 1220's. This twenty sided tower which contained living quarters a great hall, and a small chapel, it is three storeys high and was protected by a ditch, a curtain wall and a moat.

The curtain wall encloses an area of about three acres, it was defended by five D shaped towers, entry to the castle was only through either of the two gates.

The Town gate had a portcullis to protect it as well as a 'murder hole'. The other gate, the Dublin gate, has a barbican projecting from the tower. Originally the barbican spanned the water filled moat which surrounded the curtain wall and had a draw bridge which was operated from above.

It is claimed by some that the upper rooms of the gatehouse were used to house the young Prince Hal, later to become Henry V, who was left at Trim by Richard II in 1399 before his fateful return to England.

King John visited Trim Castle in 1210 and the following year the crown took control of it spending £64 on repairs and renovations.