From the Introduction:
Battering rams are among the most ancient engines of siege warfare. They are used to crack open walls or it those are too massive, as is the case here, the more vulnerable wooden gates of fortresses. The battering ram in this case is a massive wooden log, its head sheathed in iron, suspended from a sturdy wooden frame by iron chains. The frame is covered by a canopy of wooden boards to provide the personnel operating it with cover against arrow or spear volleys fired from the gatehouse or the walls above it. To ease its deployment to the gate the ram is equipped with no less than four axles with massive iron bound wheels.
Still the success of the ram is anything but given. It is uncertain if it will be even able to reach the gate equally reinforced with steel bands. It could still be set on fire by the defenders. Even if the defenders don't manage to set the ram itself on fire they could still try to torch the bridge the ram has to cross to reach the gate. If the contraption finally reaches the gate it could still be smashed by heavy boulders dropped from the battlement. And the gate itself is not only reinforced by steel bands but held closed by two massive crossbeams each one easily as massive as the ram itself.