An Aplan Mortarium, also known as a "Maze of the Dead", was a kind of crypt built by the Aplans on their home planet Alfava Metraxis.
The Mortarium was a large temple with subterranean labyrinths, with tombs in the walls and adorned with statues of the dead. The Maze had six levels, to represent the ascent of the soul. During the 47th century, these mazes became infested with Weeping Angels, leading to the downfall of the Aplan civilisation. (DW: The Time of Angels)