The mid-13th century saw the English Crown push hard to consolidate its hold on Ulster and Connacht. The Cenél nEógain - the great Gaelic dynasty of the O'Neills - had been weakened by the death of their high king Aodh Méith in 1230, and a vicious succession struggle within the family followed. In 1241 the Anglo-Norman-backed claimant Donal Mac Lochlainn was killed at the Battle of Camergi by Brian O'Neill, who had allied with the O'Donnells of Tír Conaill. The English administration in Dublin saw an opportunity. John FitzGeoffrey, the Justiciar of Ireland, launched a major campaign in 1248: he reorganised the province's administration, built a bridge across the Bann at Coleraine, and constructed Killowen Castle on the west bank as a fortified bridgehead. From this base he pushed inland against the Cenél nEógain, forced them to surrender hostages, and made peace on his own terms. He extended the campaign further by building another castle at Magh Cobha in modern County Down. Within a single year, Anglo-Norman power had reached deeper into Ulster than it ever would again.

The castle's older name was Drumtarsy - *Druim-tairsigh*, possibly meaning 'ridge of the crossing' - and the parish surrounding it took its name from the church built to serve the garrison community. St Eugene's was constructed in 1288 and the parish became *Cill Eogain*, 'church of Eugene', anglicised over the centuries to Killowen. The area was commonly called 'the Castle-side of the river' to distinguish it from Coleraine proper on the east bank. The 1213 chronicle entry that mentions an earlier English castle at Coleraine, built by Thomas mac Uchtry, describes the construction as destructive: the town's existing cemeteries and buildings - except for the church - were dismantled to provide stone. That earlier fortification appears to have been short-lived. FitzGeoffrey's 1248 castle at Killowen was the more durable structure, the one that warranted royal investment over more than a century.

By 1382, Killowen Castle was still in use but evidently in poor condition. King Richard II's administration in Dublin ordered John Rynaux, Treasurer of Ulster, to repair the castle along with the bridge of Coulrath (Coleraine) and the towers located on either side of the bridge. This is the last substantial documentary reference to the building as a functioning fortification. After the 14th century the castle vanishes from the records - not destroyed in any recorded siege, just slowly abandoned, perhaps stripped for building stone, perhaps demolished when the area's strategic significance shifted. By the time of the Plantation of Ulster in the early 17th century, when the London livery companies arrived to build the modern town of Coleraine, no one mentions the old castle at all. It had become invisible. The 'Waterside' name took over, and Drumtarsy faded entirely from local memory.

There is a fragmentary tradition that the castle in the background of the heraldic crest of Derry City may be Killowen Castle - though the link is contested, and the more common interpretation is that the depicted castle is intended to represent the Plantation walls of Derry itself. The Derry crest features a castle with a skeleton at its base, commemorating a 14th-century tale about a man named Walter de Burgh who was starved to death in his cousin's fortress at Northburgh, near present-day Greencastle in Donegal. The crest's imagery is layered with multiple medieval references, of which Killowen may be one. Heraldry, like castles, accumulates meaning faster than it can be documented. The fact that 21st-century Derry City still carries the image of a medieval castle on its civic insignia is, in its own way, a continued life for whichever castle - Killowen, Northburgh, or any of the others - the original heraldic artist had in mind.

The Killowen district of modern Coleraine - the western suburbs across the bridge from the town centre - sits over the ground where John FitzGeoffrey's castle once stood. The Killowen Primary School, the Killowen Parish church of the Church of Ireland, the rows of post-war housing along the Killowen Road - all of them named for a parish named for a church named for a saint named to serve a castle whose precise location no archaeologist can now confirm. This is how medieval Ulster persists. Not in standing ruins like Dunluce or Carrickfergus, where the stone work clearly survives, but in the texture of placenames and parish boundaries and the half-buried foundations that occasionally turn up when someone digs a new house extension. Killowen Castle is not a tourist site. There is no signpost, no interpretive board. But the memory of FitzGeoffrey's 1248 campaign survives in the very word Killowen - a saint, a church, a parish, a suburb, a deeply embedded reminder that the present is built quite literally on top of the medieval past.