This modest ruin, marked on Noble and Keenan's 1752 map as Kiltaghan Castle and later as Kyltaghan on Taylor's 1783 survey, stands as a low circular mound of collapsed masonry about 17 metres across. The castle's various names reflect the fluid nature of Irish placenames; whilst the Ordnance Survey christened it Offaly Castle in 1838, the Rathangan Manor book of 1760 preserved at Carton still referred to it as Kiltaghan Castle, after the original townland.

Today, visitors to Mountprospect townland will find little more than a grassed-over cairn rising between 0.8 and 1.6 metres high, with a single surviving section of wall stretching just over six metres. This fragment, built from roughly coursed rubble masonry about 70 centimetres thick and 1.8 metres high, retains some of its original facing stones at the western end. In 1986, surveyors noted traces of a barrel vault amongst the ruins, though these architectural details have since become obscured by time and collapse.

The castle's setting adds another layer to its historical significance, positioned at the northern foot of a long, gentle slope and immediately east of what appears to be a late medieval field system. About 75 metres to the north lies another unclassified building, suggesting this area once held greater importance in the local landscape. Aerial photographs from 1970 captured by Cambridge University provide valuable documentation of the site before further deterioration, preserving details that might otherwise be lost to memory like the castle's original name.