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Shûshtar is an ancient fortress city in the Khuzestan province in southwestern Iran.
During the Sassanian era, it was an island city on the Karoun river and selected to become the summer capital. The river was channelled to form a moat around the city, while bridges and main gates into Shushtar were built to the east, west, and south. Several rivers nearby are conducive to the extension of agriculture; the cultivation of sugar cane, the main crop, dates back to 226 CE. A system of subterranean channels called Ghanats, which connected the river to the private reservoirs of houses and buildings, supplied water for domestic use and irrigation, as well as to store and supply water during times of war when the main gates were closed. Traces of these ghanats can still be found in the crypts of some houses.
When the Sassanian Shah Shapour I defeated the Roman emperor Valerian, he ordered the captive Roman soldiers to build a vast bridge and dam stretching over 550 metres, known as the Band-e Qaisar ("Caesar's bridge").
The ancient fortress walls were destroyed at the end of the Safavid era.

(From Wikipedia)