The end of the final mission; you fully achieve Alternate History and prevent the Mongolian invasion of China by halting Ghengis Khan's army.
Most any mission where you build a monument; special mention to the Great Wall, the Grand Canal, and of course, the Terracotta Army of Qin.
The second mission of the Qin campaign; for the first time in history and in the game, all of China is united under one banner. Thus, all enemy cities ally with you or become your vassal, and you're given a sprawling map, the largest in the game up to that point, to begin building a new Qin capital.
The Song campaign's first mission then tops that; China is not united, so you have to unite it the hard way by conquering four enemy cities and bringing them into the fold one by one. And then you do it. And, according to the mission briefing, this is on the heels of your forces taking the capital of Kaifeng and deposing a Puppet King boy-emperor. So as far as the story is concerned, you conquer five cities; the first one was just a Curb-Stomp Battle not worth depicting so you can just get to your main city.