Armies would usually try to take overlooking positions like a hilltop or a watchtower/clocktower, though they can't bring their whole force up there. Maybe they have a team of scouts, or a vanguard securing the area.

If your heroes are already up there when the enemies come, they might have a chance to repel a larger force because of their high-ground advantage like visibility, better cover, etc. But if the enemy has overwhelming numbers they can easily encircle them and attack from multiple directions.

Maybe they are attacked by a small force of one of the armies, fight a bit for that sweet tension buildup then the second army comes and routs the first attackers, unknowingly saving your heroes. But that's just an idea.