The Mysterious Tower

From the website: "In this all-new adventure, the characters come across an ancient wizard's tower. The nearby keep has been reduced to rubble over the ages, but the tower is in perfect condition. It is surrounded by an impenetrable force field that cannot be breached - not even by the ghost of the long-dead wizard, who has been trapped within for centuries. Surely there must be great treasure within this magical abode. But how to get to it?" Later remade for Castles and Crusades as GG1: The Mysterious Tower.