Explain how the Norman Invasion was so successful

The Norman Invasion was so successful due, most importantly, to the preceding invasion at York, of Harald Hardrada and Tostig Godwinson. This forced Harold Godwinson to march his army north and leave his Southern Coast undefended. Combined with the Northern Invaders, Harold had already disbanded his navy (patrolling the Channel) in order to gather the harvest and the weather was so poor that he thought it unnecessary to relaunch the patrols until the new year. Thus, William faced a coastline with no defenders or defences. Secondly, whether through tactics or simple necessity, William ravaged the Earldom of Wessex, Harold's homeland, and this is believed to be Harold (a brilliant commander and soldier)'s reason for rushing his exhausted and undermanned army into battle at Hastings and not waiting longer at London for the reinforcements from his Northern Earls, Edwin and Morcar. Harold could have been attempting to trap the Normans against the Channel and not let them advance any further through his Kingdom. However he should have sacrificed his personal pride and let Wessex suffer for long enough to meet with his reinforcements.In addition to the exhaustion of Harold's army, the numbers the Anglo-Saxons lost at Fulford Gate and Stamford Bridge became all too apparent when Harold's less trained Fyrd broke ranks and chased William's feigned retreat, breaking the shield wall. The fundamental difference between the two armies was that the Normans were professional soldiers and mercenaries to a man, whereas the Anglo-Saxons was forced into mobilising their Fyrd (untrained farmers and peasants) due to their losses in the North and Harold's eagerness to surprise William.