Persian Siege Warfare and Engineering: From the Walls of Babylon to the Castles of the Sasanian Frontier
The Achaemenid army, in 547 BCE, did not take Sardis by storming the citadel. According to Herodotus (I.84), a Lydian deserter showed Cyrus’s men a path up the undefended side of the acropolis, and the city fell in a few hours. The story is almost certainly a piece of Greek literary simplification, but the technical point is right: the citadel of Sardis, on its steep volcanic outcrop above the Pactolus, was not a target for a Persian assault. It was a target for a Persian reconnaissance. The lesson generalises. Across the seven centuries and three empires of Persian military history, the technical question of siege warfare is rarely a question of breaking walls by force. It is a question of finding the bypass — the undefended path, the dry moat, the undefended gate, the river that can be diverted, the wall that can be undermined — and the engineering tradition of the Persian armies was, in the main, a tradition of bypass and exploitation rather than frontal assault.
This is the article I want to write: not a chronicle of the great sieges, but an account of the engineering tradition that made the bypass possible. The standard works in English are the relevant chapters in Pierre Briant’s From Cyrus to Alexander (2002), the Sasanian army article by Klaus Schippmann in the Encyclopaedia Iranica, and the older but still useful David Oates, Studies in the Ancient History of Northern Iraq (1968), which covers the Sasanian frontier fortifications in detail. The wider context is in the Persian Military and Warfare survey.
The Assyrian inheritance
The Achaemenid engineers, in the sixth century BCE, inherited the cumulative siege-craft of the great Near Eastern kingdoms that the empire had absorbed. The Assyrian relief programme of the ninth to seventh centuries BCE is the most complete surviving visual record of pre-Achaemenid siege warfare. The Lachish reliefs of Sennacherib (c. 700 BCE) show the full Assyrian kit: ramps, siege towers, battering rams, mines, and movable shelters. The Achaemenid engineers, recruited from the populations of the former Assyrian heartland in northern Mesopotamia, were familiar with all of these. The Babylonian kingdom, with its long tradition of urban defence and its control of the great cities of lower Mesopotamia, contributed its own tradition of moat, wall, and waterway defence. The Egyptian, Phoenician, and Jewish engineers brought their own techniques to the Achaemenid service.
The result was a formidable technical corps. The Achaemenid engineering service was responsible for the design of fortifications, the construction of siege works, the building of the Royal Road and its supporting infrastructure, and the maintenance of the bridges, canals, and irrigation works that were essential to the imperial economy. The Susa foundation charter of Darius I, on a clay tablet now in the Louvre, lists the peoples who contributed to the building of the great palace at Susa: Egyptians, Babylonians, Medes, Persians, Sogdians, Ionians, Sardians, and Jews all worked side by side on the project. The multilingual, multi-ethnic character of the imperial engineering service is visible in the building inscriptions themselves.
The Euphrates operation at Babylon (539 BCE)
The most famous siege of the Achaemenid period was the taking of Babylon by Cyrus the Great in 539 BCE. The technical content of the operation, on the most defensible reading, was the diversion of the Euphrates upstream of the city, lowering the level of the river inside the walls, and entering through the river gates. Herodotus (I.191) tells the story in some detail: the Babylonian garrison was at a religious festival, the river was diverted, the Persian troops entered through the dry river bed, and the city fell. Xenophon (Cyropaedia VII.5) gives a similar account. The Babylonian chronicle (the Nabonidus Chronicle, BM 35382 in the British Museum) confirms the broad outline: the city was taken “without a battle.”
The technical question is whether the diversion of the Euphrates was a real engineering operation or a literary device. The answer, I think, is that it was both. The Euphrates was a substantial river; diverting it required a canal cut upstream, the kind of work that Babylonian engineers had been doing for the irrigation system of southern Mesopotamia for two millennia. The Persian engineering staff at Babylon in 539 BCE included Babylonian specialists; the Achaemenid state had absorbed the Babylonian engineering corps wholesale. The river diversion was, in technical terms, a routine operation. What made it decisive was the political context: the Babylonian garrison, distracted by a religious festival, did not respond. The technical bypass and the political opportunity combined, and the city fell.
The Cyrus Cylinder, found at Babylon in 1879 by Hormuzd Rassam and now in the British Museum, records Cyrus’s subsequent treatment of the city: respect for the gods, the temples, and the customs of the inhabitants, and restoration of the cult of Marduk. The cylinder is often cited, in modern scholarship, as an early example of the Achaemenid policy of religious and cultural tolerance. The “first declaration of human rights” reading is a 20th-century construction, popularised in Iran under the Pahlavis and picked up by the United Nations when a replica was displayed in 1971. Modern Assyriologists, including Irving Finkel, treat the cylinder as a piece of Babylonian royal propaganda in a long tradition. Finkel’s re-reading of the cuneiform in 2013 is the most important modern philological work on the text.
The siege of Tyre (332 BCE)
The siege of Tyre by Alexander the Great in 332 BCE is the last great siege of the Achaemenid period — a siege in which the Achaemenid empire was the defender, not the attacker. Tyre, one of the principal naval powers of the Achaemenid empire, had submitted to Alexander but refused to allow him to enter the city and sacrifice to the Tyrian god Melqart. Alexander, unable to take the city by sea, decided to take it by land.
The technical content of the operation was a causeway. Alexander’s engineers built a causeway from the mainland to the island on which the city stood — about 600 metres across open water — using the rubble of the old city of mainland Tyre as fill. The causeway was exposed to fire from the Tyrian walls, and the operation took seven months. The siege towers and battering rams of the Macedonian engineering corps, working at the end of the causeway, eventually breached the wall. The fall of Tyre in July 332 BCE was the first major siege of the Hellenistic tradition and is often cited as the moment when the Hellenistic tradition of siege warfare decisively surpassed the Achaemenid.
I think this reading is partly right and partly wrong. The Macedonian engineering corps of 332 BCE was a substantial body of technicians, and the causeway was a real technical achievement. But the Achaemenid state had been able to maintain the loyalty of the Phoenician fleet for nearly two centuries without ever having to besiege Phoenicia, and that political fact is, in its own way, a measure of the political skill of the Persian kings. The Macedonian siege of Tyre was, in a real sense, a consequence of the political failure of the Achaemenid state, not of any technical inferiority. By 332 BCE the political logic had collapsed, and the Macedonians were doing the besieging.
The Sasanian frontier: the gird
The Sasanian empire, like the Achaemenid before it, was defended by an extensive system of frontier fortifications. The Sasanian gird — a chain of fortified cities, watch-posts, and garrison towns stretching from the Caucasus to the Helmand — is one of the most sophisticated defensive networks of the ancient world. The great Sasanian fortresses of Ctesiphon, Nisibis, Dara, Amida, and Theodosiopolis on the western frontier, and of Herat, Kandahar, and Bust in the east, were major engineering works in their own right. The hundreds of smaller fortifications along the frontier — the watch-towers, the signal stations, the garrison towns — were the working basis of the system.
The Sasanian frontier, as reconstructed from the surviving Pahlavi texts, the Roman and Byzantine accounts of the wars with Persia, and the archaeological remains in Iran, Iraq, and the Caucasus, was designed for defence in depth. The frontier was divided into a series of sectors, each commanded by a spāhbed, or general, and each supported by a network of smaller fortifications. The system allowed a relatively small Sasanian field army to respond quickly to any incursion. The Roman and Byzantine limes systems of the late empire borrowed from the Sasanian model, and the Arab military engineers of the early Islamic period inherited it.
The most thorough study of the Sasanian frontier remains the work of the Russian military historian V. F. Minorsky and the Soviet-era excavations at the Caspian gates, but the recent French-Iranian excavations at the fortress of Takht-e Soleyman, the Sasanian site in Azerbaijan that was probably the royal summer capital, are giving us a much more detailed picture of the technical sophistication of the Sasanian building programme. The Sasanian fortress of Takht-e Soleyman, on a volcanic outcrop above a deep lake, was an engineering project on a scale comparable to the Roman forts of the British wall, and the surviving foundations of the temple of Adur Gushnasp are a measure of the technical resources of the Sasanian state.
The siege of Constantinople (626 CE)
The Sasanian siege of Constantinople in 626 CE, in which the Sasanian forces were assisted by their Slavic and Avar allies, is one of the most ambitious sieges of the ancient world, and the failure of the siege was one of the turning points of late antiquity. The Sasanian army under Shahrbaraz crossed the Bosphorus from the Asian side, and the Avars and Slavs attacked the land walls from the European side. The defence was led by the patriarch Sergius and the patrician Bonus, with the young emperor Heraclius away campaigning in Persia. The defence held, and the Sasanians withdrew.
The technical content of the operation is less well documented than the dramatic content. The Avars, who were the technical specialists of the siege, brought a range of siege engines — towers, rams, and possibly early forms of the trebuchet, though the trebuchet is not securely attested in this period — and the Sasanian contribution was the investment of the city by land and the cutting of the grain supply from the Bosphorus. The failure of the siege was a combination of the strength of the Theodosian walls, the discipline of the Byzantine defence, and the political fact that the Avars and the Sasanians were not operating under a single command.
The engineers, by nation
The Achaemenid and Sasanian siege trains were multinational in character, and the technical specialisations were distributed across the empire. The Phoenician engineers, recruited from the cities of Tyre, Sidon, Byblos, and Arvad, were the most famous of the imperial engineers and were responsible for the design of the imperial fleet, the construction of the great harbours of the eastern Mediterranean, and the siege works of the western campaigns. The Jewish engineers, recruited from the communities of Mesopotamia, Judea, and Egypt, contributed to the design of fortifications, the construction of the imperial road system, and the maintenance of the imperial irrigation works; the Jewish colony at Elephantine in Egypt, well documented in the Aramaic papyri, included a substantial number of soldiers and engineers. The Babylonian engineers, recruited from the populations of the former Babylonian heartland, were the heirs of the longest continuous tradition of siege warfare in the ancient world and contributed to the design of the imperial fortifications, the construction of the imperial palaces at Persepolis and Susa, and the maintenance of the great canal system of lower Mesopotamia.
Greek fire, the famous incendiary weapon of the late Roman and Byzantine worlds, is universally attributed in the Greek sources to Callinicus of Heliopolis, a Greek-Egyptian refugee at the Byzantine court, in 672 CE. The claim that it had an Iranian component is speculative. The Sasanian use of incendiary weapons in siege warfare is well-attested — naphtha and quicklime appear in the Sasanian-period accounts of several sieges — but the specific formula of Greek fire is a Byzantine development.
What we don’t know
The hardest question in the field is the question of who designed what. The Achaemenid and Sasanian siege trains left few technical manuals, and the Greek and Roman sources, which are our main literary witnesses, are interested in the besieger’s perspective rather than the defender’s. The Sasanian fortifications of the western frontier are well preserved archaeologically, and the recent French-Iranian excavations at Bisotun, at the site of the Sasanian victory monument of Shapur I, and the ongoing work at the Sasanian fortress of Qal’eh Dokhtar in Fars are giving us a much more detailed picture of the technical tradition. But the political and the personal dimensions of the engineering service — who commanded, who designed, who made the technical decisions in the field — are, for the most part, unrecoverable.