City wall is one of the two rewards you can choose after you have upgraded a level 2 city to level 3 (the other being Resources). It puts up a wall in the city which multiplies the defense of units stationed in the city by 4.
|Warrior||Rider||Archer||Defender||Swordman||Catapult||Knight||Giant (Super unit)|
|Defense in city without City Wall||3||1.5||1.5||4.5||4.5||0||1.5||6|
|Defense in city with City Wall||8||4||4||12||12||0||4||16|