Built within what appears to be an early medieval ringwork that was later transformed into a bawn, this late 16th or early 17th century structure may have been home to Thomas Nugent, one of the commissioners responsible for the Plantation of Longford in 1620. Local historian Lewis noted in 1837 that these ruins marked the boundary of the English Pale and claimed they were besieged by Oliver Cromwell himself during his notorious Irish campaign.

The castle’s remains reveal a sophisticated defensive residence that once comprised a two-storey block with an impressive four-storey tower attached to its northern end. Though time has not been kind to the main block, with only fragments of walls standing no more than 2 metres high, the tower tells a more complete story. Its northern and eastern walls survive to wall-walk level, showcasing architectural details that speak to both comfort and defence: punch-dressed quoin stones, ogee-headed windows, gun loops, and the remains of a fine cut-stone fireplace with chamfered jambs and lamp brackets. The ground floor was covered by a barrel vault, whilst wooden floors supported by joists served the upper levels, accessed via a mural staircase in the western wall that connected to spiral stairs in the northwest corner.

What makes Coolamber particularly fascinating is its blend of defensive and domestic features. The narrow slit windows and gun loops provided protection, whilst the fireplaces, including one carried on an external chimneystack supported by corbels, offered warmth and comfort to its inhabitants. The presence of multiple doorways, now blocked, shows how the tower and main block were interconnected, creating a complex living space that could accommodate both daily life and potential siege. Located about 475 metres from a medieval church and graveyard, the castle formed part of a broader settlement pattern that characterised this contested borderland between Gaelic and English controlled territories.