The monument includes a motte, the eastern part of the inner bailey adjoining it and the remains of a stone castle, situated in the centre of the town of Eye. Also included is a 19th century mock keep known as Kerrison's Folly, constructed on top of the motte above the remains of the medieval keep. The structural remains of the castle and the 19th century folly are a Listed Building Grade I. The western part of the inner bailey, which is not included, was the site of a 19th century workhouse and is now occupied by modern housing.
The motte is visible as a conical mound 12m in height and approximately 57m in diameter at the base, with a sub-circular platform about 18m in diameter at the summit. The inner bailey to the west of the motte is ovoid in plan, wider at the western end, and defined by a scarp up to 4.5m high on the north and west sides and 2m high on the south. Limited excavations at the western and eastern ends have demonstrated that it was constructed on a natural hillock and that the interior, surrounded by an earthen bank, was raised and levelled by the dumping of imported soil to a depth of between 1.3m and 3m. The motte was probably surmounted originally by a wooden tower and the bailey surrounded by a timber palisade. These were replaced by a stone keep and a curtain wall of stone, and the ruined remains of part of the curtain wall survive on the north eastern slope of the motte and the north side of the inner bailey adjoining it, within the area of protection. The wall is visible in three discontinuous sections and is constructed largely of mortared flint rubble with some squared blocks of clunch. The longest section has an overall length of 27.2m and comprises a rectangular tower at the western end, with three narrow chambers in line adjoining it. To the east of these, at the foot of the motte, are the remains of a second rectangular tower, and near the summit of the motte, where the curtain wall would have abutted the keep, are the remains of another narrow chamber. The western tower, which projects 0.9m beyond the curtain wall externally, has internal dimensions of 2.9m east-west by 2.6m, with walls 1.6m thick and up to 3.2m high. The adjoining chambers are 1.8m wide internally and 5m, 2.8m and 3.7m long respectively from west to east. The inner and outer walls are 1.2m thick and about 1.7m in height. Only the foundations of the dividing wall between the two western chambers survive, but the dividing wall between the middle and eastern chambers still stands to a height of up to 3.9m. There is no visible evidence for windows or doors, and the chambers were perhaps intended originally for storage, although it is thought that they were used in the 14th century as a prison. The eastern tower has internal dimensions of 2m east-west by 1.6m, and the foundations, which are all that survive of the walls, are about 1.8m thick. Evidence for a chamber to the south of it was found during excavations carried out in 1987- 1988. The section of the curtain wall on the motte is up to 2.1m high and has a maximum length of 7.9m. The chamber which it contains is 3.8m in length north west-south east by 1.7m wide, and the inner and outer walls are between 1.4m and 1.6m thick. The internal walls to east and west do not appear to be bonded to the outer walls and may have been inserted at a later date. There are indications of a another chamber to the west of this, and lower down the slope of the motte, on the same alignment, is a large block of fallen masonry.
Excavations in part of the interior of the bailey adjoining the wall removed approximately 1.6m of post-medieval deposits and uncovered a layer of demolition rubble dated to the 14th century, overlying traces of a clay floor.
By the early 16th century little remained of the stone castle apart from a tower and some ruined walling, and a windmill was erected on top of the motte around 1592. A path with steps cut up to 2m deep into the western side of the motte was probably constructed to provide access to the mill and is shown on the tithe map of 1839, which also shows a mill still in existence. The mock keep was built by General Sir Edward Kerrison around 1844, it is said as a house for the batman who served him at the battle of Waterloo, and occupies almost the whole of the top of the motte. It is constructed of mortared flint with moulded brick quoins and dressings, and the shell wall is polygonal in plan, with nine sides and buttresses at the angles. Each of the outer faces of the wall is decorated with a mock loophole. Much of the wall survives to its full original height of 4.6m, but the buildings within are ruinous, standing for the most part to less than 2m. On the western side of the enclosure is a ruined tower 4m square which projects beyond the shell wall, and within this, in the north east angle, is the base of a spiral stair to a now vanished upper storey, with a hearth against the wall to the west of it. Adjoining the tower to the south and south east are the remains of two larger rooms connected by internal doorways, and against the eastern wall of the keep is the base of a detached outside lavatory. Evidence that the folly may have been built on the surviving foundations of the medieval keep was found in 1990, when a small trench was dug against the eastern wall.
Construction of the motte and bailey castle was probably begun by William Malet, who was granted the estate known as the Honour of Eye after the Conquest, and was completed by his son, Robert. When Robert Malet was banished in 1102 the estate, with the castle, reverted to Henry I and was subsequently granted to Stephen de Blois (later King Stephen). Stephen's successor, Henry II, granted it to Thomas a Becket in 1156, and it was probably Becket who was responsible for the original construction of the stone castle. After Becket's murder in 1170 it returned to the Crown, and it was sacked during the rebellion of Hugh Bigod, Earl of Norfolk in 1173. It was subsequently repaired, and regular repairs and improvements were carried out until the end of the 12th century. Henry III granted the estate to his younger brother, Richard, Earl of Cornwall whose son, Edmund, inherited. The castle was sacked again in 1265 during the Barons revolt against the King. In 1337 the estate was granted to the de Uffords, the new Earls of Suffolk, and in 1381 went to the de la Poles, but by 1370 the castle was assessed as worthless, although parts remained standing.
A modern viewing platform within the 19th century folly on the motte is excluded from the scheduling, together with modern railings, the steps up the motte, a beacon within the inner bailey, benches, information boards, floodlights and a junction box, a litter bin and the surface of a car parking area, although the ground beneath all these features is included.