"The great city of Persepolis was built in terraces up from the river Pulvar to rise on a larger terrace of over 125,000 square feet, partly cut out of the Mountain Kuh-e Rahmet. To create the level terrace, large depressions were filled with soil and heavy rocks which were then fastened together with metal clips; upon this ground the first palace at Persepolis slowly grew. …. Limestone was the main building material used in Persepolis. After natural rock had been leveled and the depressions filled in, tunnels for sewage were dug underground through the rock. A large elevated cistern was carved at the eastern foot of the mountain to catch rainwater for drinking and bathing. ... The terraced plan of the site around the palace walls enabled the Persians to easily defend any section of the front. The ancient historian Diodorus Siculus recorded that Persepolis had three walls with ramparts, all with fortified towers, always manned. The first wall was over seven feet tall, the second, fourteen feet, and the third wall, surrounding all four sides, was thirty feet high. … Because of its remote location in the mountains, however, travel to Persepolis was almost impossible during the rainy season of the Persian winter when paths turned to mud and so the city was used mainly in the spring and summer warmer seasons." Darius built Persepolis in the location he did because