Onagers are great, but @davehemke is talking about Assyria’s unique units. The battering ram and siege twoer are melee units, so you have to roll them up to go toe-to-toe with their targets.
Cities with defenses powerful enough to warrant battering rams and siege towers? :)
But, yes, in my main Assyria playthrough, the battering ram gets to stay home as a garrison. Obviously not its intended use.
-Tom