Trim Castle is the largest and one of the most important Norman military constructions in Ireland. Work began on Trim Castle around 1172 and the building was completed with the central tower in the 1220s. The three acre enclosure is dominated by a 25 metre high stone keep and is mounted on a Norman motte.

There are three levels inside, the lowest one divided in two by a central wall. The principal outer wall is around 500 meters long and remains largely standing today. It dates from around 1250 and includes eight towers and the gatehouse. Just outside the central keep are the remains of an earlier wall. The site was protected by a ditch, a curtain wall and a moat. The curtain wall was protected by five D-shaped towers and entry to the castle was only through two of the gates. Within the castle was the great hall, a chapel and living quarters.

A draw-bridge was operated through the Dublingate while the Towngate had a porticullis to protect it as well as a 'murder hole'. Extensive conservation work has been done on the castle.