Ralph Neville, Earl of Westmorland, ordered the castle built in 1430 at the bend of the River Tyne where the village of Bywell sat. The Nevilles were among the great Border magnates, and they built fortresses across the north. Bywell was meant to be one more in the chain. It was meant to be impressive. What got built was the gatehouse - a substantial three-storey defensive tower with a vaulted entrance passage, machicolations, and the kind of dressed stonework that signalled serious money. What did not get built was almost everything else. The curtain wall is fragmentary; the interior buildings never reached completion. Why the project stalled is not entirely clear. Family priorities shifted, money went elsewhere, the political weather changed. The gatehouse stood alone.

Henry VI's flight after Hexham brought him here on the night of 15 May 1464. The battlefield is about ten miles upriver; Bywell sat on the route any fugitive heading for the safety of Newcastle or the Scottish border would have taken. Henry's lords had kept him out of the actual fighting - he had been captured in battle three times already, and his protectors had grown careful - but the rout left him exposed and moving fast. He stayed at Bywell only briefly before pushing on into hiding that would last for more than a year. The bonnet and certain other personal items he left behind at Bywell were preserved as relics. The story of Henry's stay is one of the few things the gatehouse has to tell, and it tells it well: an unfinished castle and an unfinished king, sheltering each other for a single night.

Bywell passed through various hands after the Nevilles and eventually became the property of the Beaumont family, who in 1906 were elevated to the Viscountcy of Allendale. The castle remains the family's seat today, with the Grade II listed house incorporated into the surviving curtain wall section. The gatehouse itself is Grade I listed and protected as a Scheduled Ancient Monument. The castle is privately owned and not normally open to the public, which means it sits quietly above its bend of the Tyne, occasionally photographed from the river path, more often simply ignored. The village of Bywell is tiny - a population of around 380 across the whole civil parish - and the castle blends into the wooded slopes above the water in a way that hides its medieval bulk from anyone not specifically looking for it.

In 1869 a London shipbuilder launched an iron-hulled collier named Bywell Castle, presumably after this Northumbrian gatehouse. On the evening of 3 September 1878 the Bywell Castle was steaming down the Thames in ballast when she struck the paddle steamer Princess Alice, returning to London from a Sunday excursion to Sheerness with hundreds of passengers. The collier cut the Alice almost in half. The paddle steamer sank within four minutes. The disaster killed between 590 and 640 people, most of them women and children, most of them drowned in heavily polluted water at a stretch of river called Gallions Reach. It remained the worst loss of life on an English waterway for half a century. The Bywell Castle herself was lost off the Spanish coast in 1883. The name moved from a gatehouse to a ship to a Victorian catastrophe - then quietly back to a quiet bend of the Tyne.