Wall

Basic Information
As the name implies, walls are to set your city’s boundaries and protection for your city. This will allow your army to withstand attacks from an enemy. Strong and firm city walls built around the city provide significant protection against attacks and invasions. All kinds of fortified units can be built here.

Fortified Unit - Space Requirements
If you are wondering why the walls are telling you that you have no vacant space but it doesn't add up. Make sure you are calculating it correctly. First... anything being "built" will count as if it were done. This insures you do not go over the max. Second... after a battle there can be some reconciliation that actually "returns" units to the wall after a battle, but if new ones are produced in the meantime, it actually puts you in the negative vacant space. Third... units cost different amounts for each type. The following table shows you the "unit space" required for each type.

Durability
The way durability works is that if the enemy army can't destroy your walls in 40 rounds, the battle ends in a draw. The attacking units lose the battle and may suffer a loss of prestige. I have not lost any troops when losing a battle after 40 rounds. They just return home unsuccessful.

Traps
Traps are hidden devices built around your city walls. The invading army may fall into traps, regardless of their types. And each fortification unit can only be used once, with a Repair Rate of 5%.

Abatis
Abatises are very effective obstacles to stop fast-pacing enemy soldiers, such as Cavalry and Cataphract. They have a Repair Rate of 6%.

Archer Tower
An Archer Tower allows your Archers to shoot arrows towards your enemy from a distance. It has a Repair Rate of 3%.

Rolling Log
A Rolling Log can be use to sweep away a large number of attacking infantry. Each Rolling Log can only be used once, with a Repair Rate of 7%.

Rockfall
Rockfalls can crush incoming siege devices, even the ones with very high defense like Battering Rams. Each Rockfall can only be used once, with a Repair Rate of 8%.