Heavy siege tanks deploy in teams of two from Nova's factory for a cost of 800 minerals and 600 gas. Compared to normal siege tanks they have much higher HP and damage.
Spider mines are effective for warding off enemy attack waves when placed at spawn points. The graduating range ability grant the already distant attacks the longest range in the game, maxing out after 15 seconds, but they do require sight from friendly units to fire. These tanks deal good damage, but are somewhat gas inefficient meaning they tend to be most commonly used when in siege mode for anti-ground splash damage.
Siege Mode/Tank Mode
Siege Mode: Transforms the Heavy Siege Tank into siege mode where it gains splash damage, greatly increased range and damage at the cost of losing all mobility and a slower firing rate.
Tank Mode: Transforms the Heavy Siege Tank back into tank mode, where it gains the ability to move and fire at faster rate.