An onager works similarly to a catapult. These machines could be used to hurl projectiles up to half a mile (with the larger versions...possible upgrade). The Roman used them in many different ways: large boulders were flung at walls to help bring them down; many smaller rocks were used against enemy troops as a sort of shrapnel; various burning projectiles were used to try and spread fire.
The Onagers is an actual weapon used in Roman Wars. They resemble the one shown in this war game
Attachment 59925
They act like mortars but are even better because they can move.
Barbarians can pull and fire the Onager at enemy defenses.
It can also be used to protect your village
The projectiles it fires can be upgraded like the mortars
But because of its power, players can only have one
Here is an example of how it works --- skip to the 45 second to see it in actionhttp://youtu.be/ASlSZ7Vph9U

