Each restored part of the Wall adds 50 points to your Village Strength.
Maximum Wall level is 8.
Each restored Wall level gives you bonus batteries. Bonus for each restored Wall level is 20 batteries. 1 level - 20 batteries: 2 level - 40; 3 level - 60... and so on until level 8.
This is what the highest level Wall looks like:
Regards, Nexters Support Team