The Arabic and Middle Persian (Pahlavi or parsing) manuscripts on the wall states that the tower was built as a tomb for two Bavandid princes in Saljooghi Era.
The tower is located on a rocky mountain with a steep slope that makes everyone wonder how it stands there! But recent discoveries helped us with the answer when a brick step was found which decreases the effect of the angle.
Architects carved and cut through the stone to build the step at the time which is yet another proof of Persian architects’ intelligence.
Source: Iranindepth.com