Stone Tower
In front of the tomb of Artaxerxes is a well-preserved and nicely-proportioned buiilding called Ka'be Zardusht (Zoroaster's Sanctuary or Cube), which is a square structure built of blocks of white hewn limestone. Wherever knowledge is wanting, theories abound. This is true for this structure.
Theory 1: Fire Temple: Similar building have been shown on coins of a later period, which had on the roof fire altars with flames. This is, however, unlikely since a much higher tower in Firuzabad shows clearly the signs of a spiral  staircase. It is unlikely that the purpose of this structure was simply to puzzle later archaeologists and we would assume that there would have been some way for priests to get to the top. The building is specially unsuited as a fire temple.
 Theory 2: This building was used by the Zoroastrian high priest to store the holy books of the Zoroastrian faith and recite the text in front of the royal tombs. This is equally unlikely, since it is not certain that the Achaemenian kings were Zoroastrian and since the similar tower in Pasargadae, far removed from any fire altars, tombs or holy district, makes no sense in such a context.
Theory 3: This was a royal tomb of an Achaemenian king before Darius. A similar structure in Nurabad and the tower in Pasargadae could support this theory. Another foundation of similar type is also found on the other side of the Pulvar River close to Naqsh-e Rajab.
There are many other theories, none of which could give a satisfactory answer to explain the purpose of this unique building. Shapur recorded the treaty with the Romans and the successes of his government in an inscription on the left side wall of the Ka'be Zardusht. The 500,000 drachma gold ransom received from Philip the Arab for Roman prisoners is mentioned here.
The Ka'be, also called Zendan-e Suleiman (Solomon's Prison), has got only one opening in an elegant doorway at the head of a flight of stone steps opposite the tomb of Artaxerxes I. Each of the outer walls is decorated with rows of vertical slots cut in the limestone; addition ally, all but the door-wall are embellished with six recessed dummy windows of black basalt. There is a single chamber inside with a lofty ceiling spanned by four vast stones.