From the back cover:

It appears the story you were given with the old key and map had at least some hint of truth to it. Before you as a white stone tower approximately 50 feet tall devoid of any windows or doors. As you circumnavigate the towers perimeter looking for any means of entry a shimmering blue ripple outlines a door-shaped portal at the base of the tower allowing you to appear inside. As you marvel at this portal you notice the key you were given is a light with a similar glow. What mysteries lie inside the Alabaster Tower of Thracia?

Fifth Edition Fantasy #29 The Alabaster Tower of Thracia is an all-new 5E adventure intended for use with four to six 1st-level characters. The characters explore the extra-dimensional interior of a tower located in the ruins of the fabled city of Thracia. While ownership of Original Adventures Reincarnated #9 The Caverns of Thracia and Original Adventures Reincarnated #7 Dark Tower is not required to use this adventure possessing those books may lead to a more enriched adventuring experience.