IIRC levies only reinforce in friendly territory. If you have an enemy reinforcing while attacking you, they most likely have either retinues, mercenaries or a holy order.
If you click on the portrait of the character that is attacking you, you can see the total number of troops available to him. A mouse over reveals a tooltip that breaks down his army, showing how many personal levies, vassal levies, mercenaries, retinues and holy order troops he has.