A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and tended to use mechanical advantage to fling stones and similar missiles. With the development of gunpowder and improved metallurgical techniques, siege engines became firearms.

Ancient siege engines

The earliest engine was the battering ram, developed by the Assyrians, followed by the catapult in ancient Greece. The Spartans used battering rams in the Siege of Plataea in 429 BC, but it seems that the Greeks limited their use of siege engines to assault ladders, though Peloponnesian forces used something resembling flamethrowers. The Trojan Horse may have in fact been a battering ram, rather than the device described by the greek poet Homer.

The first Mediterranean people to use advanced siege machinery were the Carthaginians, who used siege towers and battering rams against the Greek colonies of Sicily. These engines influenced the ruler of Syracuse, Dionysius I.
Two rulers to make use of siege engines to a large extent were Philip II of Macedonia and Alexander the Great. Their large engines spurred an evolution that led to impressive machines, like the Demetrius Poliorcetes' Helepolis (or "Taker of Cities") of 304 BCE: nine stories high and plated with iron, it stood 40 m (125 ft) tall and 21 m (60 ft) wide, weighing 180 t (360,000 lb). The most utilized engines were simple battering rams, or tortoises, propelled in several ingenious ways that allowed the attackers to reach the walls or ditches with a certain degree of safety. For sea sieges or battles seesaw-like machines (sambykē or sambuca) were used. These were giant ladders, hinged and mounted on a base mechanism and used for transferring marines onto the sea walls of coastal towns. They were normally mounted on two or more ships tied together and some sambykē included shields at the top to protect the climbers from arrows. Other hinged engines were used to catch enemy equipment or even opposing soldiers with opposable appendices which are probably ancestors to the Roman corvus. Other weapons dropped heavy weights on opposing soldiers.

The Romans preferred to assault enemy walls building earthen ramps (agger) or simply scaling the walls, as in the early siege of the Samnite city of Silvium (306 BC). Soldiers working at the ramps were protected by shelters called vineae, that were arranged to form a long corridor. Wicker shields (plutei) were used to protect the front of the corridor during its construction. Another Roman siege engine sometimes used, resembled the Greek ditch-filling tortoise, called a musculus ("Little mouse"). Battering rams were also widespread. The Roman Legions first used siege towers around 200 BC.

The first documented occurrence of ancient siege artillery pieces in Europe was the gastraphetes ("belly-bow"), a kind of non-torsion bolt-thrower. These were mounted on wooden frames. Greater machines forced the introduction of pulley system for loading the projectiles, which had extended to include stones also. Later torsion systems appeared, based on sinew springs. The onager was the main Roman invention in the field.
The earliest documented occurrence of ancient siege artillery pieces in China was the levered principled traction catapult and an 8-foot high siege crossbow from the Mozi (Mo Jing), a Mohist text written at about the 4th - 3rd century BCE by followers of Mozi who founded the Mohist school of thought during the late Spring and Autumn Period and the early Warring States period. Much of what we now know of the siege technology of the time came to us from Books 14 and 15 (Chapters 52 to 71) on Siege Warfare from the Mo Jing. Recorded and preserved on bamboo strips, much of the text is now unfortunately extremely corrupted. However, despite the heavy fragmentation, Mohist diligence and attention to details which set Mo Jing apart from other works, ensured that the highly descriptive details of the workings of mechanical devices like Cloud Ladders, Rotating Arcuballistas and Levered Catapults, records of siege techniques and usage of siege weaponry can still be found today.

Medieval siege engines

thumb|The medieval cannon balls/" class="wiki">Mons Meg with its 20" (50 cm) cannon balls
Medieval designs include a large number of catapults such as the Mangonel, Onager, the ballista, the traction trebuchet (first designed in China in the 3rd century BC and was brought over to Europe in the 4th century AD), and the counterweight trebuchet (first described by Mardi bin Ali al-Tarsusi in the 12th century). These machines used mechanical energy to fling large projectiles to batter down stone walls. In Europe, the catapult was invented in Greece by Dionysius in 399 BC. Also used were the battering ram and the siege tower, a wooden tower on wheels that allowed attackers to climb up and over castle walls, while protected somewhat from enemy arrows. Another weapon was the petard, an explosive device designed specifically for breaching gates and walls. The petard had to be placed directly against the surface of the fortress.

A typical military confrontation in medieval times was for one side to lay siege to their opponent's castle. When properly defended, they had the choice whether to assault the castle directly or to starve the people out by blocking food deliveries, or more proactively to employ war machines specifically designed to destroy or circumvent castle defenses.

Other tactics included setting fires against castle walls in an effort to decompose the cement that held together the individual stones so they could be readily knocked over. Another indirect means was the practice of mining, whereby tunnels were dug under the walls to weaken the foundations and destroy them. A third tactic was the catapulting of diseased animals or human corpses over the walls in order to promote disease which would force the defenders to surrender, a primitive form of biological warfare.

Modern siege engines

With the advent of gunpowder, firearms such as the arquebus and cannon—eventually the mortar and artillery—were developed. These weapons proved so effective that fortifications, such as city walls, had to be low and thick, as exemplified by the designs of Vauban.

The development of specialized siege artillery, as distinct from field artillery, culminated during World War I and II. During the First World War, huge siege guns such as Big Bertha were designed to see use against the modern fortresses of the day. The apex of siege artillery was reached with the German Schwerer Gustav gun, a huge 800 mm caliber railway gun, built during early World War II. Schwerer Gustav was initially intended to be used for breaching the French Maginot Line of fortifications but was not finished in time and (as a sign of the times) the Maginot Line was circumvented by rapid mechanized forces instead of breached in a head-on assault. The long time it took to deploy and move the modern siege guns made them vulnerable to air attack and it also made them unsuited to the rapid troop movements of modern warfare.

Siege weapons are now considered obsolete owing to the effectiveness of aircraft-delivered munitions and cruise missiles, which have made defensive area fortifications obsolete. The only cost effective static defensive structures are now deep bunkers used for military command and control. Even these fixed assets are of questionable value as it appears that the most survivable command and control of mobile defensive forces (such as modern tactical and strategic aircraft, mechanized cavalry and mechanized infantry) is through decentralized command and the use of mobile command centers.