To become siege mode, they must first spread their tracking wheels apart and withdraw their mounts that hold their position from firepower's kinetic force (removed in Season 3 to simplify the siege tanks' design). They are sometimes used by terran armies to defend the base and to offend against distant zerg and protoss units and buildings.
But if the rush is extravagant, they would be outnumbered. Siege tanks started their battlefront in ' Heart of the Swarm Opening' as the terran line of defense against rushes. Siege tank provides armored terran infantry, support and mobile powerful long-ranged firepower.
