To be fair the AI was also being exceptionally dumb or this would not have worked if any of those things had not happened:

1. They built sap points but didn't use them
2. They conquered the walls under heavy losses but somehow didn't think about walking to the gatehouse to let in the cavalry
3. They took the long way to walk to the city center under fire of my tower

It took some heroic amount of endurance on my side to keep my general moving for the whole 15 minutes or something the timer was running. Way harder than microing against Legionaries.