The Ka'ba-ye Zartosht (), or the Cube of Zarathustra, is a stone quadrangular stepped structure in the Naqsh-e Rustam compound beside Zangiabad village in Marvdasht county in Fars, Iran. The Naqsh-e Rustam compound also incorporates memorials of the Elamites, the Achaemenids and the Sasanians. The Ka'ba-ye Zartosht is from the mountain, situated exactly opposite Darius II's mausoleum. It is rectangular and has only one entrance door. The material of the structure is white limestone. It is about high, or if including the triple stairs, and each side of its base is about long. Its entrance door leads to the chamber inside via a thirty-stair stone stairway. The stone pieces are rectangular and are simply placed on top of each other, without the use of mortar; the sizes of the stones varies from to , and they are connected to each other by dovetail joints. Etymology The structure was built in the Achaemenid era and there is no information of the name of the structure in that era. It was called Bon-Khanak in the Sassanian era; the local name of the structure was Naggarekhaneh which means house (khaneh) keep/hold (Naggar). The phrase Ka'ba-ye Zartosht has been used for the structure from the fourteenth century into the contemporary era. Various views and interpretations have been proposed for the purpose of the chamber, but none of them can be accepted with certainty. Some consider the tower a fire temple and a fireplace, and believe that it was used for igniting and worshiping the holy fire. Another theory is that it is the mausoleum of one of the Achaemenid shahs or grandees, due to its similarity to the Tomb of Cyrus and some mausoleums of Lycia and Caria. Other Iranian scholars believe the stone chamber to be a structure for the safekeeping of royal documents and holy or religious books, but the chamber of Ka'ba-ye Zartosht is too small for this purpose. Other less noticed theories, such as its being a temple for the goddess Anahita or a solar calendar, have also been mentioned. In the Sassanian era, three inscriptions were written in the three languages Sassanian Middle Persian, Arsacid Middle Persian and Greek on the northern, southern and eastern walls of the tower. One of them belongs to Shapur I the Sassanian, and another to the priest Kartir. According to Walter Henning, "These inscriptions are the most important historical documents from the Sassanian