"I don't know if it's the best thing to be sharing with you, but if you promise not to go telling the whole world, I'll let you in on the secret. There's a hidden ladder behind the Godric tapestry beside the fire place. It leads right up to the top landing of the girls' dorm behind the portrait of the giantess up there."
With a grin, he added, "But be careful. Once Sirius went up there and swung the portrait open, only to see McGonagall going up the steps. He shut it just in time but we think she saw him because she was scowling at him all week. Then again...she was usually scowling at him..."
He pushed a rook forward only to have one of Harry's pieces bludgeon it to death.