Tank Bunkers are structures which most turreted vehicles can enter to gain additional protection from enemy fire while still being able to retaliate. Though the occupier of a Bunker will become a much more difficult target to destroy for ground forces, aerial attacks will still easily bypass these entrenchments.
Tank Bunkers offer a unique defensive option for the Epsilon, as vehicles are required to garrison it in lieu of infantry. Despite the name, other turreted vehicles are able to enter and fire within it, such as the Stinger. Thanks to its large hit points and lack of power requirement, enemies will have a hard time destroying this structure in battle if they are not supported with airstrikes or artilleries.
As vehicles can enter and exit with no problem, the Tank Bunker is a customizable and versatile defense. Epsilon main battle tanks can deal with its rivals with efficiency, Stingers can either be a stationary detector or repair bay with an Engineer inside (though only one unit at a time can be repaired), and even appropriate mind-controlled vehicles can enter the bunker (such as Abrams Tanks or Tesla Cruisers). While they can be liberated from mind control, they will still be trapped in the bunker, forcing them to destroy it. Additionally, the vehicle inside gains a slight increase in damage, range and firing rate.
Scorpion Cell benefits from this structure the most since they have the most vehicles that are viable to enter in a Tank Bunker, and it addresses their weakness of low armor. In fact, PsiCorps and Epsilon HQ don't have any faction-exclusive units besides main battle tanks that are able to enter a Tank Bunker due to their large sizes and/or lack of turret, whereas Scorpion Cell have the Speeder Trike, Oxidizer and Plague Splatter to put their weaponry to great use (unfortunately Tyrants can't enter due to their lack of turret).
On the other hand, Tank Bunkers are situational compared to other Epsilon defenses, as proselytes usually prefer their vehicles to be mobile instead of being stationary. While the procedure of entering and exiting Tank Bunkers take little time, the issue of constant replacements due to airstrikes discourages them to build one in the first place, and instead build other Epsilon defenses that aren't disabled in a single airstrike.