How to relocate enemy city out of my territory?

It depends of how strong is the barricade of the enemy, sometimes two minutes and the city is gone, sometimes 30 min aren't enought.

When you attack a city and are victorious, you set their barricade on fire. While the barricade is burning it reduces durability. If and when you reduce their barricade durability to 0, it forces them to randomly teleport elsewhere on the map.