Strong earthquakes occur in Iran with a frequency unheard of almost anywhere else on Earth.
Quakes of 7.0 and above on the Richter scale occur somewhere in Iran about once every five years. So when you visit monuments, even those of only modest antiquity, you'll often hear a litany recounting the occasions on which it was knocked down and put back up. The southeast tower of the Citadel is an example of this. Earlier repairs to the base are evident and, to date, remediation of the tilt has amounted to patching up the resulting gap in the wall with additional bricks. Work is said to be in progress on restoring this monument.