The ancient name of Trim was Ath Truim, signifying “the ford of the boortrees” or elders, the latter half of which appellation alone remains. It is situated on the Boyne, twenty-two and a half miles north-west by west of Dublin on the Enniskillen road.
The castle lies east of the town, on the right bank of the river, and has been said to be the only castle in Ireland deserving the name. It is certainly the finest specimen of the Anglo-Norman fortress, and was one of the chief strongholds of the Pale. It occupies a sloping mound and its walls enclose a triangular area of about three acres, which measures 486 yards in circumference, and was protected by eight circular flanking towers at nearly equal distances and two gate towers.
The north-east side was formerly washed by the Boyne, but now a low meadow intervenes.
On the west or town side a gate-tower occupies the centre. The grooves for the portcullis are still perfect, and from abutting masonry it would appear that there had once been a drawbridge and barbican. The lower portion of the tower is rectangular and the upper part octagonal.
In the southern side is a circular gate tower of Gothic shape. Here also are the portcullis grooves visible and a recess for the windlass, as well as the remains of arches over the moat, and a barbican beyond.
A moat or ditch surrounded the fortress, on the two sides not washed by the Boyne, which was supplied with water by a small stream.
Where the town wall joined the castle at the south-west angle a mound of earth has been artificially raised to the height of the castle wall. This was probably done for the mounting of cannon when the castle was re-fortified in 1647.
One of the towers near this mound was cleared in 1836. It is of three storeys, and the upper part was found to be a pigeon-house, having holes for 60 or 80 pair, while the lower storey contained a postern leading to the level of the moat water. In 1425 it is recorded that the King granted the custody of the dove-cot to Thomas Brown, with pasture called the Castle Orchard.
The north angle of the bawn contains the ruins of several buildings, one being of three storeys with a high gable, and four large windows in the wall towards the river. The piers for the arched roof are still to be seen, and it is likely to have been the banqueting hall or chapel. A large vault extends under part of it. In one of the towers near this the mint is supposed to have been situated. Another tower contains the remains of a small chapel.
The keep consists of a rectangular figure, on each side of which a small square tower abuts, thus forming a twenty-sided figure from which missiles could be showered in all directions. The walls of the centre tower are twelve feet thick, while those of the adjoining ones measure from four to six feet. Winding stairways lead to the summit, at some 60 feet from the ground, while on each angle of the centre tower is a square turret nearly 17 feet in height.