There is abundant evidence to indicate that Troy, and the Trojan war, didn't happen in the Mediterranean at all, but in northern Europe, perhaps as a struggle of early Celts; and that Homer was merely reciting his version of an older conflict, the news of which has passed widely throughout ancient Europe. I suggest a reading of the book Where Troy Once Stood, by Iman Jacob Wilkens, for a good exposition of an alternate interpretation of the particulars of that conflict.
Whether there really was a Trojan War is disputed. Maybe something similar happened in Celtic Europe (not unlikely given they were all a bunch of bloodthristy Indo-European tribes), but the Greeks would have known nothing of anything in northern Europe. It's possible there was an older story about a siege that happened to be applied to the ruins at Troy after Greeks settled in the area.