"Well...through the doors, Harry." -- Albus Dumbledore
The antechamber is a small room connected to the Great Hall, door behind the teachers’ table, with a fireplace and many portraits, including that of the Fat Lady‘s friend Violet (GF17).
"Harry went through the door out of the Great Hall, and found himself in a smaller room, lined with paintings of witches and wizards. A handsome fire was roaring in the fireplace opposite him" (GF17).