From A Wiki of Ice and Fire
The Turtle is a type of crude siege engine used by the wildlings to attack Castle Black in the Battle of Castle Black. It is described as a "a hull turned upside down and opened fore and aft; a longhall on wheels".
The Turtle has a rounded top and eight huge wheels with a stout wooden frame underneath and a number of layers of raw mammoth skins on top of sheepskins and pelts. It is eventually destroyed by the men of the Night's Watch, who drop barrels of frozen gravel on it from atop the Wall.