Siege Weapons can be used at special siege pads located around the keep and around the bottom of the keep. These are crucial for attacking and defending a keep. There are four types of siege weapons that can be purchased from the merchants at the Warcamps near the Keeps.
Rams: can be used against the Keep Doors to knock them down. Up to 4 players can sit in a Ram at any time and damage dealt by the ram is reflected on how many players are present. Each player contributes 25% of the rams total damage. In other words if 4 people are in the ram, it does 100% damage. The ram also protects you from 50% of all incoming damage. Take that oil!
Ballistae: can be used against individual enemy players or other siege weapons. The Ballistae has a Sniper mode, which is the mode you can accurately aim and fire in. Use this to target and kill enemy players.
Cannons, Catapults and Trebuchets: can be aimed to fire at a selected location. It will do AoE damage to enemy players and enemy siege weapons in the area. Cannons, Catapults and Trebuchets cannot shoot through buildings roofs or walls. That means if you're under a roof, behind a wall or in a building you won't be effected by the AoE damage done outside.
Boiling Oil: can be used to rain death down to anyone knockin' on the front door. This pot is very important for defense.