Built as a rectangular fortress measuring 8.7 metres north to south and 10.4 metres east to west, with walls two metres thick, this structure represents the defensive residential architecture favoured by Gaelic and Anglo-Norman families during Ireland's turbulent medieval period. The castle's most notable feature is perhaps the datestone of 1575 embedded above the fireplace on the second floor, providing a rare precise date for the building's construction or significant renovation.

The castle's defensive features reveal the paranoia and pragmatism of its builders. Entry through the east wall immediately subjects visitors to a murder hole overhead, whilst a guard room in the northeast corner and spiral stairs in the southeast angle control movement through the building. The interior layout is surprisingly sophisticated, with a series of small chambers stacked vertically above the murder hole, accessible from the main rooms on each floor. The ground floor chamber, lit by three ogee-headed windows with decorative spandrels, showcases the same punch-dressed stonework found at the mint in Carlingford, County Louth, suggesting shared craftsmen or architectural influences across Ireland during this period.

Living conditions in the tower house balanced comfort with security; multiple garderobes (medieval toilets) were built into the walls at various levels, accessed through narrow mural passages, whilst fireplaces on the upper floors provided warmth. The second floor appears to have been the principal living space, crowned by a barrel-vaulted ceiling that supported an attic level above. Though the third floor is now destroyed, evidence of its former existence remains in the form of ruined fireplaces and chimney stacks at the wall walk level. The building's windows tell their own story of evolving military technology, with traditional ogee-headed openings supplemented by later additions of circular and cruciform musket loops, marking the transition from medieval to early modern warfare. Protected under a preservation order since 1937, Coole Castle remains an evocative reminder of how Ireland's minor nobility once lived; fortified, functional, yet not without refinement.