Air level of the map.

Let's say, during a siege at the castle walls, when the walls and gates are breached, the defending army has the opportunity to retreat into the city limits. By choosing to retreat, the besieged army falls back to the second line of defense – let's say, behind barricades – and the battle action takes place in the narrow city streets. In an even worse situation, they can retreat to the last line of defense – the castle hall, separated from the city by strong walls and gates.