A siege tower or breaching tower (or in the Middle Ages, a belfry) is a specialized siege engine, constructed to protect assailants and ladders while approaching the defensive walls of a fortification. The tower was often rectangular with four wheels with its height roughly equal to that of the wall or sometimes higher to allow archers or crossbowmen to stand on top of the tower and shoot arrows or quarrels into the fortification. While the towers were primarily constructed of wood and thus flammable, their builders typically incorporated whatever non-flammable coverings could be readily procured including iron and/or fresh animal skins.

Evidence for use of siege towers in Ancient Egypt and Anatolia dates to the Bronze Age. They were used extensively in warfare of the ancient Near East after the Late Bronze Age collapse, and in Egypt by Kushites from Sudan who founded the 25th dynasty. During classical antiquity they were common among Hellenistic Greek armies of the 4th century BC and later Roman armies of Europe and the Mediterranean, while also seeing use in ancient China during the Warring States Period and Han dynasty. Siege towers were of unwieldy dimensions and, like trebuchets, were therefore mostly constructed on site of the siege. Taking considerable time to construct, siege towers were mainly built if the defense of the opposing fortification could not be overcome by ladder assault ("escalade"), by mining, or by breaking walls or gates with tools such as battering rams.

The siege tower sometimes housed spearmen, pikemen, and swordsmen or archers and crossbowmen, who shot arrows and quarrels at the defenders. Because of the size of the tower it would often be the first target of large stone catapults, but it had its own projectiles with which to retaliate.

Siege towers were used to get troops over an enemy curtain wall. When a siege tower was near a wall, it would drop a gangplank between it and the wall. Troops could then rush onto the walls and into the castle or city. Some siege towers also had battering rams which they used to bash down the defensive walls around a city or a castle gate.

In the First Intermediate Period tomb of General Intef at Thebes (modern Luxor, Egypt), a mobile siege tower is shown in the battle scenes. In modern Harpoot, Turkey, an artistically Akkadian style stone carved relief dated circa 2000 BC was found depicting a siege tower, the earliest known visual depiction from Anatolia (although siege towers were later described in Hittite cuneiform writing).

Siege towers were used by the armies of the Neo-Assyrian Empire in the 9th century BC, under Ashurnasirpal II (r. 884 BC – 859 BC). Reliefs from his reign, and subsequent reigns, depict siege towers in use with a number of other siege works, including ramps and battering rams.

Centuries after they were employed in Assyria, the use of the siege tower spread throughout the Mediterranean. During the siege of Memphis in the 8th century BC, siege towers were built by Kush for the army led by Piye (founder of the Nubian 25th dynasty), in order to enhance the efficiency of Kushite archers and slingers. After leaving Thebes, Piye's first objective was besieging Ashmunein. He admonished his assembled army for their failures, and took personal command. He had a wooden siege tower erected from which his archers could overlook the city and fire down on the enemy.

During the siege of Syracuse in 413 BC, Athenians erected a siege tower on ship hull. Alexander did the same at Tyre (322 BC) as did Marcellus in Syracuse (214 BC). Towers were used against both land and naval targets. At the time of Julius Caesar, ship towers were built with a lighter, collapsible design that could be stowed flat on the deck when not in use, lowering the center of gravity.