Why Didn't Severus Disapparate In the Shrieking Shack?

I do believe that Severus knew he was in danger in the Shrieking Shack. Harry even wondered if he sensed danger. Voldemort said to him,

I have thought long and hard, Severus ... do you know why I have called you back from the battle?

If you put two and two together, Snape must have know he was in danger. He must of know that the fight was almost over, because Dumbledore told him, to tell Harry, (indirectly) the message about Harry being a Horcrux. He had orders, so why could he just Disapparate into the castle?