The Following description of the battle comes from Wikipedia :
Edwin had brought some soldiers to the east to prepare for an invasion by the Norwegians .The battle started with the English spreading their forces out at Germany Beck to secure their flanks. On the right flank was the River Ouse, and on the left flank was the Fordland, a swampy area. The disadvantage to the position was that it gave Harald higher ground, which was perfect for seeing the battle from a distance. Another disadvantage was that if one flank were to give way, the other one would be in trouble. If the Anglo-Saxon army had to retreat, it would not be able to because of the marshlands. They would have to hold off the Norwegians as long as possible.
Harald's army approached from three routes to the south. Harald lined his army up to oppose the Anglo-Saxons, but he knew it would take hours for all of his troops to arrive. His least experienced troops were sent to the right and his best troops on the riverbank.
The English struck first, advancing on the Norwegian army before it could fully deploy. Morcar's troops pushed Harald's back into the marshlands, making progress against the weaker section of the Norwegian line. However, this initial success proved insufficient for victory to the English army, as the Norwegians brought their better troops to bear upon them, still fresh against the weakened Anglo-Saxons.
Harald brought more of his troops from the right flank to attack the centre, and sent more men to the river. The invaders were outnumbered, but they kept pushing and shoving the defenders back. The Anglo-Saxons were forced to give ground. Edwin's soldiers who were defending the bank now were cut off from the rest of the army by the marsh, so they headed back to the city to make a final stand. Within another hour, the men on the beck were forced off by the Norwegians. Other invading Norwegians, who were still arriving, found a way to get around the thick fighting and opened a third front against the Anglo-Saxons. Outnumbered and outmaneuvered, the defenders were defeated. Edwin and Morcar however, managed to survive the fight.
York surrendered to the Norwegians under the promise that the victors would not force entry to their city, perhaps because Tostig would not want his capital looted. It was arranged that the various hostages should be brought in and the Norwegian army retired to Stamford Bridge, 7 miles east of York, to await their arrival.