This rectangular stone fortress, measuring roughly 15.5 metres east to west and 8.8 metres north to south, stands on what appears to be the eastern edge of an earlier Anglo-Norman motte and bailey complex. The castle’s substantial walls, between 1.6 and 2 metres thick and built from random rubble without any dressed stone, survive to two storeys in height, with remnants of a high stone vault still visible over the ground floor.

The castle’s interior reveals fascinating architectural details that help date it to the 14th or 15th century. Entry was through a doorway in the western wall, protected by a drawbar slot, whilst narrow slit windows provided light to the vaulted ground floor chamber that stretched over 12 metres in length. A mural staircase, its outline still traceable in the eastern wall, once connected the ground floor to the upper levels. The building’s design suggests it may represent a transitional style between the earlier first-floor hall houses of the 13th century and the later multi-storey tower houses that became popular in the 15th and 16th centuries. Adjacent to the north side of the castle, earthworks and a large depression mark the site of what was described in 1826 as a ‘mansion house’, which may have served as a gatehouse to the castle courtyard.

For centuries, Castlelost was the seat of the Tyrrell family, Anglo-Norman lords who held considerable power in County Westmeath. In 1486, Maurice Tyrrell granted the advowson of the nearby parish church to St Mary’s Abbey in Dublin, whilst in 1552, Thomas Tyrrell of Castlelost held extensive lands throughout Fertullagh by knight’s service. The castle’s alternative medieval names, ‘Veteri Castro’ (old castle) and ‘Cremato Castro’ (burnt castle), hint at a turbulent history. The Tyrrells’ fortunes came to an end during the Confederate Wars of the mid-17th century, when Thomas Tyrrell’s estates were forfeited and subsequently granted to Protestant landowners Margaret Aggas and Sir Jeremy Alexander. Today, the castle stands as a remarkable survivor of medieval Ireland, its weathered walls and collapsed vaults bearing witness to centuries of conflict, changing ownership, and the eventual abandonment that left it to the elements.