Herod must have been allowed to keep some troops of his own. He had to have the sort of "muscle" so he could collect taxes, persecute real and imaginary opposition, etc.
But the real power behind him which set him in power and kept him were Roman troops. According to Josephus, when the Romans set Herod as king they made a great slaughter that he had to plea them to stop so else he would have no subjects left to rule over.
Being a usurper, a tyrant, and an Edomite convert, there is no doubt that the only think that kept the people from overthrowing him is the Roman army.