Air level of the map.

For example, the first arena could be the siege, the second the inner part, and the third the castle hall.

Let's say, during a siege at the castle walls, when the walls and gates are breached, the defending army has the option 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 takes place in the narrow city streets. If the situation is even worse, they can retreat to the last line of defense – the castle hall, separated from the city by strong walls and gates.