Inferi plod through the front doors as Death Eaters swarm past, casting the Killing Curse as they go. The Ministry ground forces fall back inside, as the bombing unit does what it can to stem the flow of the walking dead. Dementors glide throughout the battle field, and more than one Auror is lost to their kisses.
The ghosts of Hogwarts flit through the halls, carrying messages and information from the severed lines of Ministry. Unlike owls or other runners, they can't be killed. Since Apparation is not possible - that charm at least has not been broken - they're the quickest way.
This castle was once a place of learning and laughter. Now it is a place of blood and terror.