Cineva spunea...
I think I got the plot....There is something hidden in constantinople...I think Altair keeps something(may be an artifact) there when the fourth crusade(1202-1204) happened(crusade was headed by the Pope). The Byzantine empire was there during that period. But in 1453, the Ottoman's raided constantinople and seiged it. Thus Ottomans got control of thar artifact. During Suleiman's period, Ezio finds about that artifact, gets it and brings it back to the Villa.(remember? Desmond had said Ezio was back to Villa for something but he didnot know what it was).

Thus we are going to play as Altair, find what the artifact is, then play as Ezio retrieve it and get it in the present as Desmond. Sounds convincing to me.