A Siege Unit is an advanced late game unit, that is capable of dealing large amounts of damage either against primarily structures and to a lesser extent enemy units. They are typically slow, unwieldy, expensive, require being unpacked/packed and very vulnerable to melee attacks. When used correctly they are devastating, against enemy defenses and troop concentrations.
A Siege Ram is typically available to all civilizations, they are essentially melee units that need to be up against structures in other to demolish them being very well protected against pierce damage they excel at taking down Fortresses and city gates. A Siege Catapult typically outrange most defenses and can quite easily wear down towers, walls and Fortresses at their leisure, but will need a compliment of defensive troops guard against attack. A Bolt Shooter is an anti-personnel siege unit that provides accurate and deadly ranged fire against units, showering deadly steel bolts against armored troops. Siege Towers are a unique unit available only to the Macedonians, which act like an armored land ship capable of garrisoning troops to provide them protection but also increased arrow fire.