How to do that again?

I was playing in a game few days ago and i was losing.. i didnt wall my cities and capitals but when some one started to attack me i thought i should make walls now ,however, i walled a city and at the same turn he attacked it after the turn passes he took the city but he was trapped by my wall his units lost a turn because it was walled by my 3 militias and i could destroy his units and won


when i tried to make this wall trap again in other games i failed i want to know:

what is the best time to make a wall around a city which is about to get attacked and the result is city walled with enemy units trapped inside ?? or its just about luck??

~movement are prioritize
~making unit in cities are not reinforcement
~movement first start when u take out unit from cities or moving them if they were outside of the cities already
~to do that trick you must move the 3 militia out of the city and try to wall them AND they must be one of your earliest move the earlier the better. DO NOT move them last or you won't make a wall and they could of been destroyed when an enemy take the city.

Thnx bro i made it: