Persepolis is near the small river Pulwar which flows into the Kur (Kyrus). The site is marked by a large 125,000 square meter terrace, partly artificially constructed and partly cut out of a mountain, with its east side leaning on Kuh-e Rahmet ("the Mountain of Mercy"). The other three sides are formed by Retaining Walls, which varies in height with the slope of the ground. From 5 to 13 meters on the west side there is a double stair, gently sloping, which leads to the top. To create the level terrace, any depressions that were present were filled up with soil and heavy rocks. They joined the rocks together with metal clips.
Gray limestone was the main material used in building Persepolis. To reach the top terrace, the construction of a broad Stairway, 20 meters above the ground, was planned to be the only main entrance. This was begun around 518 BC. The dual stairway, known as the Persepolitan stairway, was built in a symmetrical manner on the western side of the Great Wall. The 111 steps were 6.9 meters wide with treads of 31 centimeters and rises of 10 centimeters. Originally the steps were believed to have been constructed to allow for nobles and royalty to ascend by horseback, new theories suggest that this was to allow visiting dignitaries to in fact walk up the stairs while keeping a regal appearance, permissible by the ease in which the stairs could be climbed due to the small distance between each step.
The top of the stairways led to a small yard in the northeastern side of the terrace, opposite the Gate of Nations. After natural rock had been leveled and the depressions filled in, the terrace was prepared. Major tunnels for sewage were dug underground through the rock. A large elevated water storage tank was carved at the eastern foot of the mountain. Professor Olmstead suggested the cistern was constructed at the same time the construction of the towers began.
The uneven plan of part of the foundation of the terrace acted like a castle whose angled walls enabled its defenders to target any section of the external front. Diodorus writes that Persepolis had three walls with ramparts, which all had towers to provide protection space for the defense personnel. The first wall was 7 meters tall, the second, 14 meters and the third wall, which covered all four sides, was 27 meters in height, though no presence of the wall exists in modern times.
