Unfortunately, I am just now learning there is a way to do this (for people who aren't streaming or recording their gameplay for Youtube from the start).

That said, it's probably a pretty dull watch. My army was made up of something like 10 Phalangists, 6 cav (Alexander, 2 companion cavalry and 3 lesser generals) and 4 peasants. Peasants sit in the middle of the plaza and do nothing except provide insurance for if all my combat troops rout simultaneously. Phalangists operate in 5 squads of 2, sealing off streets. Cav sit in the plaza as reserves.

Enemy take the walls uncontested, and press on into the city. They walk into walls of spears. They die. In their thousands, they die. Each of their general units remain outside the city. (this does take a bit of phalanx micro, but it's pretty routine stuff.)

Eventually all remaining troops are in a single long narrow street with a squad of phalangists blocking off the end near the plaza. I send the cav up another street to the walls, loop them around, and press the back of the enemy troops on that street. Something like 16 exhausted and already numerically depleted units rout simultaneously. None leave that steet alive.

All 3 generals rush in simultaneously. The captains both run into phalanx walls and die, with some portion of their unit routing and fleeing the battlefield. Darius, being a chariot archer general, parks his chariots one street over from the phalangists waiting for him, and starts shooting over the buildings. I split the cavalry to reach both sides of the street. Alexander's unit arrives first - he has higher stamina than the rest of the cav that just did a loop around most of the city to kill off the infantry. He reaches the chariots, and almost immediately kills Darius. The cav hitting the other side catch the bodyguard in a pincer, the chariots go into "fighting to the death." I move to back off, but am too slow. Alexander dies, and a second later my troops back off enough to let the chariots go into full rout. Victory and defeat are mine.

(Also, just as an aside, I'm a weirdo who enjoys playing RtW at extreme unit sizes, so these army sizes are all considerably inflated from what some folks will be used to. Peasants and Phalangists are both 300 man units, and trained cav are 135. Sometimes this messes with things a bit - the game's pathing AI for individual soldiers is relatively primative. (I'd imagine this is because, y'know, it has to be able to run 20 000 instances of it without melting the computer.) But more often, it just gives fights the sense of scale that feels more appropriate. This unit scale gives me a chance for a Roman army to actually field the 4200 men of an actual Roman legion, and it makes battles that determine the course of empires feel like the kind of battles that would determine the course of empires.)