Northumbria had been tearing itself apart. Aelle had driven out the previous king, Osberht, by force. Then a Viking army landed in East Anglia in 865 - the chroniclers call it the Great Heathen Army, the largest Scandinavian force yet seen in Britain - and on 1 November 866 it took York. The chroniclers say Ivar, who later became known as Ivar the Boneless, was avenging the death of his father Ragnar Lodbrok. The Vikings had little trouble with the city. The Anglo-Saxon civil war was their gift. They took Eoferwic; they failed to take Aelle. Through the winter the two rival Northumbrian kings did something rare in any era: they put aside the war over the crown and joined forces to take their capital back.

What happened on 21 March 867 reads in the chronicles as a battle in two acts. The Northumbrians attacked and at first they made progress, breaking through the Roman defences and pushing into the streets. Then the Viking experience told. The chronicles say the narrow streets nullified any advantage of numbers - and the geography of medieval York would have been a maze of timber buildings, alleys, market squares and dead ends, every one of them held by warriors who had spent the winter learning the ground. A different account holds that the Vikings were briefly trapped between the Northumbrian attackers outside the walls and the Northumbrian garrison inside, before rallying. Either way, the rally happened. The Anglo-Saxon Chronicle puts it bluntly: "Both kings were slain on the spot."

Norse tradition gave the death of Aelle its own particular horror. Sagas written centuries later say Ivar and Ubba, fighting as brothers, captured Aelle alive and put him to the blood eagle - a ritual execution in which the ribs are cut from the spine and pulled outward to form the shape of wings, the lungs drawn out to flap as the victim died. Whether this actually happened is one of the long-running debates of early medieval scholarship. The blood eagle appears only in late Norse sources and may be poetic invention, a literary revenge for Ragnar Lodbrok rather than a historical execution. The Anglo-Saxon Chronicle, much closer in time, says only that both kings were killed in the fighting. The truth is buried somewhere in the streets of Jorvik.

What followed was less dramatic but more consequential. The Vikings set up a puppet king named Ecgberht, an Anglo-Saxon willing to take orders. He lasted until 872, when a revolt sent him into exile in Mercia. Halfdan Ragnarsson - another son of Ragnar - ended the rising in 876, occupied southern Northumbria directly, and divided the land among his followers. Out of this grew the Kingdom of Jorvik, the Scandinavian York that would dominate the region under varying degrees of Viking control until 954. By that point the language of York was already mixing Norse and English in ways still audible in Yorkshire speech a thousand years later - the place-name endings in -by and -thorpe, the street-names in -gate from Old Norse gata. Coppergate. Stonegate. Micklegate.

Stand in central York now and almost every layer of this story is under your feet. The Roman walls still survive in stretches - the Multangular Tower in the Museum Gardens is the same Roman masonry the Northumbrians and Vikings climbed over. The medieval city walls were built on top. The Viking workshops and houses excavated at Coppergate between 1976 and 1981 are reproduced inside the Jorvik Viking Centre, where you can ride past them in a small car and smell, more or less, what a Viking street smelled like in the autumn of 975. The streets that confused Aelle's men in 867 are still the streets you walk on. York buries nothing properly. It just builds the next thing on top.