User blog comment:Animusradiation/Ubisoft E3 Conference Liveblog!/@comment-115.134.210.222-20110607082348/@comment-208457-20110607100824

If one wanted to sail from Rome to Masyaf, you wouldn't go via Constantinople(Istanbul). You would travel to the east coast of italy and sail around Greece, then straight on to the west coast of Syria.

Also, we've seen that Ezio breaks his hidden blade in Masyaf, we know that he gets the hookblade as a replacement. I suspect that Ezio tried heading for Masyaf, found it was too well protected, so retreated to Constantinople.