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. There is a tower-like circular building on the top of the mound. Locals have given this structure the name Burj-e Gurban (Tower of Sacrifice) and it is believed to have been a watchtower.
Honestly it is not worth going up the mound. the path is not secured and it's very steep and there are no safty for the ones who want to go up there. However, if you decide to give it a shot don't expect something special up there, but you will have a nice view of the city and the mountains around it from this point. Since, they are charging for entrance here least they can do is to secure the path and put some signs up there !