The Town Wall gives all your defending troops a defense bonus. It is very useful if you know how to use it. A Town Wall which is not properly scaled with your Town Hall could work against you giving an attacking bonus to your enemies! In order to optimize the wall you have to check both the Town Wall level and the Town Hall level, according to the formula:

% Def = (10 * WallLvl^2) / TownHallLvl

Where:

Def is the defense bonus in % over the defending troops.
TownHallLvl is the Town Hall’s level.
WallLvles is the Town Wall’s level.
From this formula you can easily see how it will do you no good to keep over upgrading your Town Hall while forgetting about the Town Wall, since the defense bonus % will shrink with each upgrade. On the other hand, if we make the extra effort of upgrading them together, we will enjoy a nice defense bonus equal to the shared level *10 in % over the defendig troops. In other words, if both the Town Wall and the Town Hall are in level 1 your troops get a 10% bonus while defending, if both are at level 2, the bonus would be 20%

We can see all combinations in this table graphically:

CityWall / TownHall Level Defense Bonus Relationship
# 1T 2T 3T 4T 5T 6T 7T 8T 9T 10T
1Wall 10% 5% 3% 2% 2% 1% 1% 1% 1% 1%
2Wall 40% 20% 13% 10% 8% 6% 5% 5% 4% 4%
3Wall 90% 45% 30% 22% %18 15% 12% 11% 10% 9%
4Wall 160% 80% 53% 40% 32% 26% 22% 29% 17% 16%
5Wall 250% 125% 83% 62% 50% 41% 35% 31% 27% 25%
6Wall 360% 180% 120% 90% 72% 60% 51% 45% 40% 36%
7Wall 490% 245% 163% 122% 98% 81% 70% 61% 54% 49%
8Wall 640% 320% 213% 160% 128% 106% 91% 80% 71% 64%
9Wall 810% 405% 270% 202% 162% 135% 115% 101% 90% 81%
10Wall 1000% 500% 333% 250% 200% 166% 142% 125% 111% 100%

Holes in the wall - Ikariam
In Ikariam a Town Wall has as many holes as its level. So a level 5 Town Wall could possibly have a total of 5 holes in it. In order to make a hole though your Town Wall, the attacking forces need to count with “Battering-Ram” units. That special units with that ability. The units that can break though walls are:

Rams
Catapults
Mortars
Each hole in your wall gives the attacking forces an extra 10% attacking bonus. From this one can deduce that if your Town Wall is not upgraded at least to your Town Hall level; your enemy could use your own Town Wall agains you!

Example:

Your Town Hall is at level 6 and your Town Wall is only level 2, by looking at the above table we can see how we only enjoy a 6% defending bonus for our troops. Because the wall is at level 2, it can host up to two holes, that is a 20% attacking bonus for your enemy. 20% - 6% yields a final 14% bonus for your enemy.

Source

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