The Isfahan Fire Temple also known as Atashgah is a Sassanid-era structure, which stands on a mound of the same name along with the remains of several other buildings. Several buildings in this cluster have a classic char taq “four arch” design, which was a characteristic of fire temples of the time, and others have been suggested housed priests and pilgrims. One part of the complex, on the southern flank of the hill, are the remains of a citadel of about twenty buildings (or rooms within buildings), many of which are however only evident as foundation traces —particularly those in the lower half of cluster. Several buildings in the cluster have a classic “char taq” (four arch) floor-plan; the main characteristic of Zoroastrian fire-temples of the 3rd century on wards and that are the actual atashgahs that housed sacred fires. Other buildings include what may have been storage rooms and living quarters for priests and affluent pilgrims. Another feature of the complex is the remains of a tower-like circular building on the very top of the same hill. This structure, which was once at least twenty meters high, is known by the local populace as the “Burj-i Gurban, or Burj-i Kurban” (Tower of Sacrifice) and appears to have been a military watch-tower with a flare that could be lit to warn of an approaching enemy (i.e. a beacon). In both cases, the remaining walls are of baked brick, held together with a clay-reed mixture. In the 10th century, the buildings were used by the Esmaili inhabitants of Isfahan to hide from tax collectors. The Arab historian Masudi visited the site around the same time, and records local tradition as having believed that the site was converted from one of idol worship to one of fire by “King Yustasf” when he adopted the religion of the Magi.