If you are open, and have a high hero assigned to Training, walls can be helpful. Reinforcements of opposite race will be killed before your walls are hit for a same-race attacker.
If you are closed, walls are a major liability and can be killed with minimal might loss by the attackers.
If you want to keep farmers wags out of your city, a small number of walls will usually work. 10 traps or as many as 25 of each t1 wall defense should keep the nuisances out.
If someone wants in, building larger walls just uses up your resources and loses the might you worked to build. The might kill ratio on un-defended walls can be as high as 40-1. The attacker may lose 100 might, and you will lose 4000.
If you want tks (might killed, not troop units), you can find a closed city with walls and kill them. They're asking for it anyway (see above).