Walls

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.

History
Before the establishment of centralized governments capable of supporting strong, national armies, each individual city was left to fend for itself. As a result, many of the cities constructed city walls to protect against raiders and bandits. City walls represented a major investment in both time and materials, requiring years to complete and constant repairs to maintain their strength and integrity. The walls helped turn a city into a fortress capable of withstanding all but the most determined attacks.

Durability
The way durability works is that if the enemy army can't destroy your walls in 100 rounds, the attacking units suffer a loss, the attacking Hero loses 5 Loyalty and the attacking units return home.
 * I have not lost any troops when losing a battle after 100 rounds. They just return home unsuccessful.

Fortified Units - 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 ensures 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.

Note that the construction times displayed are greatly reduced if Construction research has been done at the Academy.