The sturdy limestone structure, built with carefully laid courses and dressed corner stones, once served as the main entrance to what was likely a substantial medieval fortification. Its survival, whilst the rest of the castle has vanished, makes it a particularly valuable piece of Dublin’s medieval heritage.

The tower’s defensive architecture reveals the practical concerns of its builders. The entrance passage features a flattened barrel vault, whilst above sits a single chamber accessed via a spiral staircase tucked into the northwest corner. Military features abound; narrow slit openings pierce the walls on multiple sides, their splayed design allowing defenders to observe and fire upon approaching threats whilst remaining protected. The eastern wall contains a pointed segmental arched recess, possibly a blocked doorway from an earlier phase of construction, alongside another tall, narrow defensive opening.

Small details hint at daily life within the tower. At the junction where the stair turret meets the main gate, there appears to be a garderobe chute; a medieval toilet facility that would have emptied outside the walls. The tower’s plan mirrors itself on either side of the gateway, creating a symmetrical defensive structure that would have controlled access to the castle complex beyond. Historical records from writers like Grose in 1791 and Ball in the early 1900s help piece together the tower’s story, though much about Simmonscourt Castle itself remains lost to time.