This rectangular tower house, dating from the 15th or 16th century, once belonged to Walter Tyrell, who was recorded as an 'Irish papist' in 1641. The Down Survey of 1657 noted it simply as 'upon Symonstowne stands a castle', and by the time of the 1913 Ordnance Survey map, it was already marked as being in ruins within its large rectangular bawn.

The castle's limestone walls still rise to an impressive 12 metres at their highest points, with the north and east walls best preserved. The structure originally measured about 14 metres by 7.6 metres, featuring a barrel vault over the ground floor; remarkably, some of the original wicker centring used during construction remains visible in the walls and doorway. A mural staircase in the northwest corner once connected the vaulted basement to the upper floor, which was likely divided into two rooms. Various defensive and domestic features survive, including small loops that light the staircase, a garderobe chute in the northeast corner, and a latrine in the west corner of the upper level.

Though heavily covered in ivy and with small trees growing from the first floor, the castle reveals fascinating construction details to those who look closely. The north wall shows a distinctive base batter added halfway up its height, whilst the thick walls, measuring up to 2.3 metres in places, contain the remains of arched windows and a pointed arch opening high in the east wall. Within the bawn area, you can also spot ruins of two 19th century buildings; one just south of the castle and another outside the bawn to the northeast, said to have been a herdsman's house from when the site formed part of a later estate.