If the enemy chose to occupy the empty fortress, they likely rolled all of their supplies in promptly, as those would be the primary thing to protect. So, even if a portion of the army was left outside and survived, they would be without food and water. Some scout units would most definitely survive, unless the invader commander chose to ignore surveying the surroundings before the morning. However, a bunch of scouts wouldn't naturally matter that much, though it would be handy, for the MC's side, if the army simply seemed to disappear entirely, leaving the kingdom totally baffled and aghast.