Great idea! but what happens if your city is undefended, or your defenders are killed in the attack? should these monsters be able to occupy the town for a while, or stay put until driven out? i think there should be some damaging affect the to the city its self during the raid. perhaps generated randomly, a city tile is destroyed. maybe a loss to money, materials, or people?
Well, I would expect that your citizens would "scatter into the country side for awhile" and the monsters/ nomadic raiders would leave once they had stolen whatever they thought was valuable. At which point, your citizens would return. This might manifest itself ingame simply as the raiders entering the town, stealing gold, food, and killing X number of citizens, and then leaving (productivity of the city would be reduced to 0 for a certain number of turns to simulate the absence of your citizens.)