This is the central section of the artwork by Pauline Baynes.

It features a scene with a lone rider - a Ringwraith, or maybe Gandalf.  In the middle- distance is a circular city or fortress with a tall tower at its centre - Isengard and Orthanc perhaps.  Beyond are more mountains and a river, and what could be another tower - the Tower of Cirith Ungol.  This panel could then represent the journey described in the second first volume of The Lord of the Rings - The Two Towers.

Each panel of the slipcase is made from a separate piece of board.  The two sides and the back panel are covered by the artwork printed on cartridge paper. The top and bottom panels are covered with black linson, as is the inside of the slipcase.