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Offensive

An attacker's first act in a siege might be a surprise attack, attempting to overwhelm the defenders before they were ready or were even aware there was a threat. This was how William de Forz captured Fotheringhay Castle in 1221.

The most common practice of siege warfare was to lay siege and just wait for the surrender of the enemies inside or, quite commonly, to coerce someone inside to betray the fortification. During the medieval period, negotiations would frequently take place during the early part of the siege. An attacker – aware of a prolonged siege's great cost in time, money, and lives – might offer generous terms to a defender who surrendered quickly. The defending troops would be allowed to march away unharmed, often retaining their weapons. However, a garrison commander who was thought to have surrendered too quickly might face execution by his own side for treason.

As a siege progressed, the surrounding army would build earthworks (a line of circumvallation) to completely encircle their target, preventing food, water, and other supplies from reaching the besieged city. If sufficiently desperate as the siege progressed, defenders and civilians might have been reduced to eating anything vaguely edible – horses, family pets, the leather from shoes, and even each other.

The Hittite siege of a rebellious Anatolian vassal in the 14th century BC ended when the queen mother came out of the city and begged for mercy on behalf of her people. The Hittite campaign against the kingdom of Mitanni in the 14th century BC bypassed the fortified city of Carchemish. If the main objective of a campaign was not the conquest of a particular city, it could simply be passed by. When the main objective of the campaign had been fulfilled, the Hittite army returned to Carchemish and the city fell after an eight-day siege.

Disease was another effective siege weapon, although the attackers were often as vulnerable as the defenders. In some instances, catapults or similar weapons were used to fling diseased animals over city walls in an early example of biological warfare. If all else failed, a besieger could claim the booty of his conquest undamaged, and retain his men and equipment intact, for the price of a well-placed bribe to a disgruntled gatekeeper. The Assyrian siege of Jerusalem in the 8th century BC came to an end when the Israelites bought them off with gifts and tribute, according to the Assyrian account, or when the Assyrian camp was struck by mass death, according to the Biblical account. Due to logistics, long-lasting sieges involving a minor force could seldom be maintained. A besieging army, encamped in possibly squalid field conditions and dependent on the countryside and its own supply lines for food, could very well be threatened with the disease and starvation intended for the besieged.

To end a siege more rapidly, various methods were developed in ancient and medieval times to counter fortifications, and a large variety of siege engines was developed for use by besieging armies. Ladders could be used to escalade over the defenses. Battering rams and siege hooks could also be used to force through gates or walls, while catapults, ballistae, trebuchets, mangonels, and onagers could be used to launch projectiles to break down a city's fortifications and kill its defenders. A siege tower, a substantial structure built to equal or greater height than the fortification's walls, could allow the attackers to fire down upon the defenders and also advance troops to the wall with less danger than using ladders.

In addition to launching projectiles at the fortifications or defenders, it was also quite common to attempt to undermine the fortifications, causing them to collapse. This could be accomplished by digging a tunnel beneath the foundations of the walls, and then deliberately collapsing or exploding the tunnel. This process is known as mining. The defenders could dig counter-tunnels to cut into the attackers' works and collapse them prematurely.

Fire was often used as a weapon when dealing with wooden fortifications. The Roman Empire used Greek fire, which contained additives that made it hard to extinguish. Combined with a primitive flamethrower, it proved an effective offensive and defensive weapon. A sallying out might also occur with such weapons, or if the siege was of a location on a coastline, from ships launched from the harbor of the location.