Facing the cliffs of Naqsh-i-Rustam and their royal tombs stands the Ka'bah-i-Zardusht, which was probably built in the first half of the sixth century B.C. This square tower, forty-one feet high and twenty-four feet square, rises from a terraced platform. It is constructed of large blocks of limestone joined without mortar and held together by means of iron cramps. Stone steps lead up to the entrance, which opens into a large single room. Scholarly opinions about the purpose of the Ka'bah are divided. Some think that it was the burial place of an early Achaemenian king; others, that it was later used as a fire temple of the goddess Anahita, where the Sasanian kings were crowned.

In 1936 excavations of the tower uncovered a Sasanian trilingual inscription, in Middle Persian, Greek, and Parthian, that Shapur I had had engraved on three sides of the structure. In it he described his three victorious campaigns against Rome (between 243 and 260 A.D.). Scholars today accept this description as historical fact. It is also an important record, for this was the last time that Greek was used in Iranian inscriptions.