The Decline of the Roman Empire
By the time everyone's favorite assassin, Ezio, gets to Rome, the place has already mostly gone to shit. Climbing around the ruined Colosseum in AC: Brotherhood is certainly better than being led around by tour guides in real life, but we'd rather experience Rome at its height—and during its fall.
The exact time of and reasons behind the fall of Rome are still debated by historians and video game reviewers, but most agree that the German Chieftain Odoacer's deposing of Emperor Romulus Augustus in 476 AD was the final nail in Rome's coffin. Of course, that single event was preceded by centuries of political intrigue and barbarian invasions.
The era is perfect for some action/adventure gameplay. We can't even decide whether we'd rather play as a Roman or a barbarian; we've heard you make more money as a Roman, but you have more fun as a barbarian.