The tower at the tip of Hook peninsula is something else entirely: a medieval cylindrical keep, 24.7 metres tall with walls nearly four metres thick at the base, which has been guiding ships into Waterford Harbour without significant interruption since the early thirteenth century. The black-and-white striped exterior is now familiar enough, but underneath those later markings is a piece of Norman military architecture that was functioning as a navigational aid before most of Europe's great cathedrals were finished.

The tower was probably raised between around 1210 and 1230, most likely at the instigation of William Marshal the elder, a powerful Anglo-Norman magnate who had survived a shipwreck off the Wexford coast and subsequently founded the Cistercian abbey of Tintern nearby as an act of gratitude. Marshal is thought to have applied the same architectural vocabulary here as at Pembroke Castle in Wales, where he built a comparable circular vaulted keep around 1200. The monks of the local community, who had adopted the Augustinian rule and were known by this period as St. Saviour's, were charged with maintaining the light; a document from 1245 records them being commanded to continue doing so. Twelve acres were set aside to support both the monks and a secular custodian. After the suppression of the Knights Templar in 1314, ownership passed to the mayor and Corporation of New Ross, who walked the bounds ceremonially to demonstrate possession as late as 1687. The light lapsed sometime around 1641 and was recorded in the Down Survey of 1656 to 1658 as a former lighthouse, formerly whitewashed, before being restored in 1671 by Robert Readinge, who enclosed the coal brazier in glass. Ownership passed to Henry Loftus of nearby Loftus Hall following a successful legal claim in 1688, and his family leased the lantern, stairs, and coal store to the Revenue Commissioners from 1706. The lamp was converted to whale oil in 1791, gas in 1871, paraffin shortly afterwards, and finally electricity in 1972, with full automation following in 1996. The distinctive striping, now two black and two white bands, replaced an earlier scheme of three red stripes applied around 1864.

The internal structure rewards close attention. The groin-vaulted floors, each supported by four ribs formed with plank centring, are the work of a skilled mason operating firmly within the tradition of military architecture. The ground floor chamber, still blackened from centuries of use as a coal store, retains a fireplace and a small antechamber; tucked into the vaulting above is a small chamber measuring roughly 3.5 by 2 metres, possibly an oubliette, a concealed cell of the kind sometimes used for prisoners, whose original means of access has not been identified. The mural staircase winds anti-clockwise through the wall thickness, interrupted at the first floor by a cluster of small antechambers, some of which are thought to have been garderobes, medieval latrines with chutes running down through the wall to a stone-lined drain that was discovered in 1978 and traced all the way to the cliff edge. Archaeological testing around the tower has uncovered the robbed-out remains of a stone enclosure wall and a pit cut into bedrock containing thirteenth-century pottery, including sherds of Ham Green ware, a Bristol-made ceramic type common on high-status sites of that period. The medieval structure and the keepers' houses have been open to visitors since 2000.