King Harold II, having abandoned his wait for the expected Norman invasion in the south of England due to an unexpected landing in the north by a huge veteran arm of vikings under their feared KIng, Harald Hardraada, speed-marched his huge, mostly-mounted (huscarls), heavily-armed army north 180m to York in just 4 days and nights with a brief night-stop at Tadcaster, moved straight through York, posting huscarls guards at every city gate to prevent leakage of his presence.
He aimed to completely surprise and massacre the fierce Norse army (only hours before York- thus the entire north?- caved in to Hardrada, and also there learning from local magnates of where the Norse army were?) then dash back south before William could land- then waiting on the Normandy coast for a favourable southerly wind. The Norsemen were then spending idle time waiting for the York VIP's/hostages & resting - UNARMOURED- in the late Summer heat on both sides of the Derwent's riverbanks.
Hardraada had been so flushed with victory at Fulford Gate only five days before, that he fatally didn't post scouts far outfield to reconnoitre for any unexpected enemies (And he allowed his warriors to leave their armour with their fleet at Riccall)- odd for such an experienced and wily veteran, perhaps?
Consequently, the first thing they knew of any Saxons in the locality was a huge cloud of road-dust and glinting metal in the distance towards York at Gate Helmsley 1m away (like a "sheet of ice" acc. to Sturlasson)- could it be the York embassy approaching?
With horror, Harald & Tostig realised they had been almost ambushed by King Harold himself whom they had thought was 180m away to the SOUTH and, ordering fast-riders to hasten word to his fleet guard some miles away to join him asap, Hardrada also ordered the troops on the west bank to fight a delaying action(or were they simply caught out after rustling cattle for food, then relaxing?), whilst he hastily formed his east-bank main army into a huge circle ready for Harold's fearsome huscarls, archers and fyrdsmen.
Charging straight into the 'east bank' Vikings(forming a crude semi-circle flanked by the Derwent at it's ends), Harold's huscarls used (or were ordered to use) their mounted advantage and the English vanguard tore into them (as I believe), hacking down many & then aimed to seize the bridge(then a wooden planked crossing, wide enough only for two men) and cutting most of them off, until the Saxon infantry caught up with their foes- the Norsemen & English disintegrated into a seething mass of struggling men and, with the bridge choked with men, many Norse were driven back into the river where they drowned.
One large chainmailed Norseman stood alone on the western end of the bridge (acc. to Scandinavian legend- but not in English sources, oddly), killing any Englishmen with his axe who neared him, holding up Harold's advance.
If this story is true, then Harold, perhaps momentarily admiring this bravery, must have ordered his archers, infantry & cavalry to hold back from killing this 'hero' there and then, maybe seeing that Hardraada/Tostig had already formed up a fierce defensive formation across the river up the slope anyway, thus too late to catch them exposed even if he killed this lone warrior immediately?
Hardrada's main army had been bought enough time, & they roared their lone comrade on, but the hero was eventually felled by an enterprising Saxon who used a swilltub down the riverbank(hiding under the hanging tree canopies?
His feat is even today celebrated in York's "Spear Pies") and impaled this hero between the legs thru the bridge's wooden footboards. Harold's army then poured across the bridge and formed up (cavalry to the fore, infantry behind?) as the archers 'covered' his advance and rear.
A parlez is supposed to have taken place with the Norse main army from the river up the slope- Harold personally offering his brother his life (& Northumbrian Earldom?) if he switched sides, (he must have been tempted, but could not face the shame of betraying Hardrada who had sponsored this invasion, any more than he might not trust his brother- whom he still wanted to avenge).
But then, what might the earls Edwin and Morcar think to that, when they and their supporters heard this later?
Tostig refused his offer, whether he actually meant it or not, and the defiant Norse roared their refusals back at Harold's men, so battle began again... huscarl against viking.
Harold must have realised that his army now would be fighting slightly uphill with their back to the river!
Just one Viking charge like the English one at Fulford five days before and...disaster for him?
Harold's mounted huscarls (if Sturlasson- writing much later- isn't confusing Stamford with Senlac?) charged up to the dense circular Norse wall of locked shields & upward thrust spears and discharged their own spears and quickly wheeled about, then repeated, to wear their foes down (the Norse front two ranks crouched down behind interlocked shields, aiming their own spears at the rider's chests, making it v.impossible to engage them, whilst those standing behind aimed theirs at the horses chests) whilst archers and spear-throwers were raining missiles upon the cavalry from inside Hardrada's circle.
This continued indecisively, the Norse thinking it rather half-hearted of the Saxons, but on it raged on. Hardrada- who had been inside the circle to plug any gaps in his wall of men- led a fierce charge with a force of retainers(as at Fulford)seeking eternal heroic fame(?) broke rank and charged the Saxons as they withdrew to regroup, as was that tactic, felling many Saxons, whilst the remainder of his army held formation and fought on behind.
As they became exposed in the open outside of their 'circle' with their ferocious counter-attacking assault, the English cavalry and archers whom they dashed through/into showered them with spears from all directions, slaying a great many.
It might have looked as though the English were about to be routed, but just at this point- Hardrada was hit in the throat by a chance arrow among many, felling him along with most of those men with him.
A second parlez?? Maybe another offer of peace by Harold during the lull as the armies reformed again(did the houscarls now dismount?), who needed all his men alive and well to return south asap?
But the Norse roared with defiance- urged on by new leader Tostig, who also was soon afterwards slain as the huscarls and fyrdsmen clashed on foot with the defiant Norse. An even fiercer phase now raged- bloodier than before, the armour-less Norse being butchered by the 100's- the Saxons taking huge casualties while doing so- as the grim day gave way to slaughter on 'battle flats'.
"Orri's Storm". At this point, the armoured yet exhausted Norse fleet-guard, led by Orri Eystein (prospective son-inlaw noble of Hardrada) arrived and crashed heavily into the tangled saxon/vikings from the S.East (many Norse collapsed & died of exhaustion just running to battle, others threw off their chainmail only to be slain), almost battering Harold's Saxons backwards/sidewards with it's initial sheer momentum of force & ferocity, the English only just held their lines & checked the fresh Norse army to a standstill.
A new and bitter fight raged- "Orri's Storm" (Norse sources state this was "the fiercest of all"), dreadful, close-quarter butchery continued "until nightfall" - both sides slugging it out in a ferocious bruiser- the Saxons slain in great numbers but eventually the depleted Norse were finally broken & routed all the way back to their ships(and locally- by Harold's 'cavalry'?) in the darkness.
Many were slain by axe & sword as they fled during the night, many others were trapped and burned alive in barnhouses where they hid, etc. At great cost to Harold's army, they had won a stunning victory.
Aftermath. So heavily slaughtered had the Norsemen been that only 24 ships out of their original 300 sailed home(over 90% of their army!) -after Hardrada's son, Prince Olaf(actually became a peaceful Norse ruler), swore peace and gave hostages and plunder, then the pitiful 1000 shellshocked survivors sailed off north to the Orkneys, collecting the Norsemen at Holderness, Scarborough and Cleveland en route.
• Orderic Vitalis noted that even in his day (12thC) their bones could still be seen in heaps.
• Geoffrey Gaimar, writing at the time, said “nobody could count a half of those left on the field”