The Wall is the main defensive structure of your city, providing you with the means to deploy armies and protect your Citadel. If an enemy attacks your city, they will have to defeat the army deployed to the Wall to plunder your city. If they're victorious, you will lose a share of your unprotected resources and your medals, your Citadel will be set on fire. If you allow it to burn, you may end up with a razed city that will impose negative penalties on construction, research and other stats, and may take a while to repair.
Upgrading the Wall increases the capacity (weight) of the army you can deploy. This capacity can also be temporarily increased by building a Statue of Defense.
Tapping on the Wall allows you to deploy an Army, use related consumables (flasks, shields, etc) and more.
You can receive additional rewards for keeping your Wall manned at all times without losing defensive battles in your city. Tap on "Prize List" to find out more.
Requirements: Barracks, House