When Walls come online it becomes significantly more difficult to take cities with normal units (even with siege-oriented support). And along rolls the lumbering Catapult to aid them in breaking the walls.
The Catapult is technically the first war machine ever invented (if we don't count the bow, of course). Unlike other passive support facilities, like the Siege Tower, it can attack on its own, hurling big rocks to reduce enemy fortifications to rubble in only a few turns, while also sporting a (relatively) decent defense.
The Catapult deals full damage from afar, unlike the other units that are available at the time, and without taking any damage in return. However, it needs to first get in range of the city to be able to hit it. Note that there isn't a "Deploy" action anymore as in Civilization V, so whenever you have a full turn available, and are in range, you can start bombarding the target!
Siege units are not completely disallowed to move and shoot in the same turn until they earn Expert Crew promotion. In fact, in order to shoot, a siege unit is required to have 2 Movement left, meaning it can move and shoot in the same turn without Expert Crew if starting its turn next to a Great General or is used on Roads.
Civilopedia entry Edit
No one is quite sure when or where someone figured out it was a lot easier to use a machine to hurl a big rock, but it is known that the Greeks were using catapults as early as the 3rd Century BC. In point of fact, the term catapult comes from the Greek for “downwards” and “to toss.” Catapults use tension in a twisted rope and/or bent wooden cross-arms to throw missiles a fair distance; since they are relatively inaccurate, catapults worked best against large, slow-moving targets (such as towns and forts). As history progressed, catapults were constructed such as to be able to be transported in pieces and assembled before a battle or siege. The last time they were used in warfare was during the early stage of WW1, when the French used them to launch grenades at the German trenches. Nowadays, they are used just for hurling pumpkins and other edibles in annual competitions among geeks.