Why is Siege weak in Game of Thrones: Dragonfire?

Siege is weak against EVERY troop type — Cavalry, Shields, Archers, Spearmen. In a pitched battle it takes hits and doesn't return many. So you don't use it to win an army fight.

What Siege does have is a unique speciality: it deals extra damage to durability, the hit points of tiles and structures. It's the tool for knocking down walls and buildings, not for killing dragons.

Worth noting: the types that beat Siege on the wheel get +7% against it. The wheel says so explicitly — Spearmen are "+7% vs Cavalry and Siege", and so on. So Siege is both weak on offence AND easy to counter.

Simple rule: Siege breaks structures. For actual combat, pick one of the four real troop types according to the counter wheel.