Originally designed as an impressive rectangular fortress with circular towers at each corner, only the southwestern portion survives today, with two of its massive round towers still rising five storeys high at the south and western corners. The castle’s strategic position on the southeast side of the main street places it within 60 metres of an earlier Anglo-Norman motte and bailey castle, suggesting Delvin’s long history as a seat of power in medieval Ireland.

Historical records paint a vivid picture of the castle’s former grandeur and gradual decline. The 1659 Down Survey described it as a “Greate Castle only the walls thereof standing”, noting that Delvin was once a thriving market town and the chief seat of the Earl of Westmeath. By 1682, Sir Henry Piers wrote admiringly of its “ancient magnificence”, describing it as “a large oblong square castle high raised” with corner towers that equalled or surpassed the main structure in height, though by then it stood “wholly waste, without roof or inhabitants”. The castle’s classification as a hall-house type fortification by modern scholars, with possible later additions including a second floor, reflects its evolution from medieval stronghold to aristocratic residence.

The surviving structure measures approximately 11.7 by 15 metres, with walls 2.5 metres thick; testament to its defensive origins. An intriguing later chapter in the castle’s history is revealed by the brick vaulting on the lower floor, which divides the space into four cells with two additional chambers in the tower bases. This 18th or early 19th-century modification suggests the castle may have served as a prison during this period, adding another layer to its complex history. Today, while the northern and eastern portions have been lost to modern development, including a house and shop, the remaining towers and walls continue to dominate Delvin’s streetscape, standing as a monument to the town’s medieval heritage and the power of the Nugent dynasty.