Mischief Maker wrote:

And why would a city surrender if 4 deer wandered into its streets?

6.3.8 Battle Fatigue & Stalemate
After round 75, all units start taking Battle Fatigue
damage. Damage is applied against each unit on
the battlefield and starts from 1d2 armor negating
magic damage on round 76. On round 77 the
damage is 1d3 AN magic and the die increases
by 1 on every subsequent combat round. The
last army with surviving units on the battlefield
wins combat.
This means that battles between very large
armies may last long enough for battle fatigue to
kick in and everyone will begin suffering damage.
The Battle Fatigue mechanism also ensures that
it is impossible for units that are mutually immune
to each other’s attacks to cause the game to be
caught in an infinite loop of unresolvable combat.
If such units face off in combat, the battle will be
resolved by battle fatigue damage after round 75
and the most likely winner is the monster with the
highest HP.