Vanilla and Rise and Fall
The Siege Tower is the second ancient siege-support unit, along with the Battering Ram. It affects melee units (and also anti-cavalry units in Gathering Storm).
The Siege Tower is the opposite of the Battering Ram in the sense that units will do major damage to the city and minor damage to the wall, whereas the ram allows major damage to the wall. In the first case, melee units are able to climb over the wall, ignoring it completely, while in the second case they use the ram to ignore the wall's natural strength which protects it from conventional weapons.
Note that these two support units do not increase damage at all, but they remove penalties.
After the June 2019 Update the Siege Tower becomes useless against anything more advanced than Medieval Walls. This practically forces attackers to use Catapults and their upgrades once the Renaissance Era hits, since they become the only thing effective against wall-defended cities in the middle game. This state of affairs will persist all the way to the Modern Era when airplanes and balloons provide additional ways of attacking cities from outside their defensive perimeters.
Climbing walls up ladders - while the defenders above hurled boiling oil, rocks and pointy objects down - wasn’t conducive to high morale among the troops, and so the Neo-Assyrians invented siege towers around the 9th Century BC. At the time, most of the towns in Mesopotamia were surrounded by mud-brick walls, generally sufficient protection against attackers, but the siege towers of the Assyrians allowed them to get over the walls relatively unscathed; thus their empire spread virtually unchecked for three centuries. It was an idea whose time had come, and soon enough all the ancient civilizations had siege towers in their arsenals. In simplest form, a siege tower was a covered wooden tower on wheels; once pushed near the offending wall, a gangplank was dropped to bridge the gap and happy warriors rushed into the town or castle. Made ever more sturdy and efficient by those master engineers the Romans, the siege tower was used throughout the Medieval Era, until gunpowder finally made them obsolete.