NOT In the history never happened that an army left the town when an enemy army is close.They would defeat you if you would leave from the town when they are so close.This option is not good.It happened ,but the army that was protecting the town left days before the enemy could arrive not when the enemy was 20 miles close.




1And it is not fair:you have your faction leader blocked in a city and a powerful enemy wants to attack him and you run and escape.
2The computer could abuse this option and you will never get a really good siege.




Maybe you should send 1/2 of your army and with the rest you will escape.