And basically destroying Kings Landing.

In this case the "enemies" aren't at the gates, they are inside the gates surrounding the Red Keep. Even if Tommon and Cersei called for reinforcements the Faith Militant could kill all the Gold Cloaks and other Lannister soldiers inside the walls and shut the gates to keep any approaching army out long enough to storm the castle and kill Tommen, Cersei, and all other nobility.

Cersei would have to do what the Mad King wanted to at the end of his reign, burn the city and all of its inhabitants to the ground in order to win this battle.