Like the other dedicated siege weapons, rams aren't built from any building and do not exist as an individual unit. They are instead constructed by the attacking army directly on the battlefield after having laid siege to an enemy settlement. Building a Siege Ram costs 50 build points.
When the attacking army assaults the settlement, any ram in the army is assigned to an infantry unit that will push it. Pushing a ram substantially reduces a unit's speed and prevents it from running.
Rams serve one single purpose: to bash down the gates of the enemy settlement in order to allow other units to come in. When a ram breaks a gate, it will be abandoned by the pushing unit, which will from then fight as normal. If the pushing unit is wiped out, another can take the ram and continue pushing.
Rams can catch fire and become destroyed if constantly attacked by enemy missile units.
As they have no other siege weapon other than the rare Fokkalista, the Darknut Legion uses a special ram as their main tool against walls. This ram is much more durable than a normal ram and can never get burned.
The Gohma utilize a unique armored siege ram composed of Gohma Chitin, with two bladelike protrusions emerging from the back.