The Wall is the city's front line of defense.
You can upgrade the Wall to increase its durability.

When the city is under attack, the Garrison Hero will lead all the troops in the city into battle.
After defeating all Garrison Troops and defensive buildings in the city, the city begins to burn.

The burning city gradually loses durability, and when its durability reaches 0, the city will be automatically teleported to a random location on the map.