This rectangular earthwork, classified as a bastion type fortification, remains invisible at ground level; its form only reveals itself in aerial photographs. The site’s most intriguing feature is what appears to be an angle tower, first documented by Leo Swan during an aerial survey in 1972.

The fortification represents a particular style of defensive structure that emerged in Ireland, characterised by its geometric earthwork design rather than traditional stone construction. Bastion fortifications, with their angular projections, allowed defenders to provide flanking fire along the walls, a military innovation that became increasingly important from the late medieval period onwards. The rectangular layout with a possible angle tower suggests this was a planned defensive site, though its exact date and historical context remain unclear.

Archaeological interest in the site peaked following Swan’s 1972 aerial photograph, which captured the earthwork’s outline when conditions were optimal for revealing such features. Subsequent aerial surveys by the Geological Survey of Ireland failed to detect any cropmarks, highlighting how ephemeral the visibility of these earthworks can be, dependent on factors like soil moisture, crop growth, and lighting conditions. The site was formally recorded in the Archaeological Inventory of County Offaly in 1997, ensuring its protection as part of Ireland’s archaeological heritage, even though it remains hidden to casual observers walking the fields of Annaghmore.