I want to say the 2nd Crusade, but I could be wrong there
Fourth Crusade, 1204, but apart from that, dead right. The city fell in 1453 because a jealous and otherwise occupied Europe would not send enough troops...the Italians only sent troops (too late) for the money they expected to get, and on the condition that the Byzantines convert to the Catholic church. The Turks broke through on their final desperate assault, and wouldn't have had a chance had an Italian fleet and a couple of thousand more professional soldiers been there.
The initial expansion of Islam was largely halted due to their inability to take Constantinople in the 8th century.

Ironically, Byzantium is the only faction i haven't tried yet on the grounds that it's too strong...maybe time for a rethink. I should trust games developers more...!