What they are actually looking at is the surviving fragment of a Cromwellian artillery fort, one of three star-shaped earthworks thrown up in 1651 to strangle the town of Galway into submission. Star forts, sometimes called bastioned forts, were designed with angular projecting corners so that defenders could cover every face of the wall with overlapping fire; this one was roughly square, about 28 metres from rampart crest to crest, with bastions at each corner. Of the three forts that once formed a continuous line of siege-works across the ridge, stretching from the Terryland River to the Lough Atalia shoreline, only this northern example retains any visible surface remains at all.

The fort was built by Parliamentary forces during the siege of 1651 to 1652, and Galway surrendered in April of the following year. The works were then abandoned, repaired again in 1690, and became briefly relevant once more during the Williamite wars. The Jacobite defenders chose not to occupy them against the advancing Williamite army in 1691, though the Williamites may have used the position as a base camp for their final assault. After the town fell, the surrounding lands were divided among the victors, and a man named John Bollingbrook acquired the eastern outskirts. His name passed to the lands, and eventually to the fort itself. A 1651 pictorial map of Galway shows the structure as a quadrangular, stone-faced earthen fort, and Ordnance Survey maps confirm it survived largely intact into the late nineteenth century, before gradual attrition reduced it to its present state by the mid-twentieth century. Excavations carried out in July 2000, ahead of a proposed car park extension, recovered a striking range of material from inside the fosse, the external defensive ditch, which was found to be 4.8 metres wide and 2.3 metres deep. Finds included musket balls, coins, buckles, horseshoes, stirrups, a cannonball and a possible spearhead, as well as large quantities of animal bone and pottery. A hearth near the centre of the fort appears to have served both as a cooking area and, given the concentration of lead shot nearby, probably as a place for casting musket balls. The most unexpected discovery was a fragmented Bronze Age urn or vase embedded in the subsoil at the north-eastern end of the fort, suggesting the ridge had drawn human activity long before any Parliamentarian engineer set foot on it.