Tostig was the brother of King Harold and had been the Earl of Northumbria until the Northumbrians revolted against him. Tostig went into exile to Flanders but in May of 1066 he invaded southern England with a sizable invasion fleet. His invasion plans failed and he sailed north, first to Scotland and then to Norway and the court of Harald Hardrada. Tostig pursuaded Harald Hardrada to assist him invade England. In September 1066 Harald and Tostig along with a large invasion fleet sailed up the River Humber and headed for York. On September 20th at Gate Fulford just a couple of miles south of York the English, lead by Earls Edwin and Morcar, fought the invaders. The Saxons were defeated by the invaders.
Five days later King Harold fought the invaders at the battle of Stamford Bridge and both Tostig and Hardrada were killed. After the battle King Harold had to march his army down to the south coast to fight William of Normandy at the battle of Hastings.
This diagram shows the route that William and his army took from Pevensey, where they landed, to London.A direct route to London was not possible so William had to circle the city and enter from the north.