From the readings and documentaries I have seen about the battle, it is fairly clear that the initial retreat of the Bretons, wether fiegned or not, did illicit a crisis on the left flank of the Norman force. Almost all the accounts mention that William felt he had to rally his "retreating" flank with his personal presence. It was in dong so that the pursuing Anglo-Saxon infantry were then cut down. It is my surmise that the Bretons were really retreating, but this is only conjecture.
After seeing the success of this unforseen result, William came to the conclusion, as astute leaders are apt to do, that he might be able to induce part of Harold's army to do so again. In this he was successful, as more and more of the shield wall began to lose its cohesion, especially on Harold's right. At some point he had to shift himself and his housecarls to shore up this area. It was then that the archers were able to add to the confusion, causing casulaties to the ever thinning infantry line by altering thier trajectory to a high angle. To this was added combined arms forays to keep up the pressure all along the line.
Finally the line started to break up from the pressure as it became too thin at some points. It was then that the cavalry could, much as modern day mounted police do, move into the gaps created and further separate and isolate individual groups. It was such a situation that Harold and his housecarls found themselves in near the end of the battle. The "arrow" incident is indeed an uncertainty. Some scholars believe that it was added to the figure of Harold in the famous Bayeau Tapestry as a kind of "propaganda" device to show how Harold deserved his fate. Others think that it was one of his bodygaurds as previously mentioned. I believe this myself, as the tapestry agrees with the written accounts of how he was so badly mauled that his body was unidentifiable except to his widow who knew how to identify him by some other means.
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