After Third World War, a cheap, effective defence was desired by the Soviets. The answer came with the battle bunker. This simple bunker could only be garrisoned by Conscripts (actually, by any friendly infantry capable of garrisoning buildings; if the Soviet commander had access to G.I.s or Initiates, they could enter too) and could hold five of these. These defenses were very effective, as they not only could fend off infantry with ease, but tanks and other vehicles as well, due to the immense volume of fire; individual bullets merely dent armor, but a large stream eventually breaks through. These simple defensive structures were first used during the Psychic Dominator Disaster.
Due to alterations in reality precipitating from the removal of Einstein from the timeline, these bunkers were no longer constructed through the Construction Yards by the time of War of the Three Powers. Instead, Soviet Combat Engineers who were given the basic tools needed to construct it in a cost efficient manner.
They required only a Soviet Construction Yard's existence to be produced, and did not require power to function or consume it. As they could also be repaired manually (as opposed to garrisoning civilian buildings, which could only be repaired by Engineers) and could be garrisoned until their destruction, they were significantly more effective than normal civilian structures ("Urban Combat" or UC buildings). It is also effective against Yuri Primes who will try to use the tactic of mind-controlling defense structures (and then selling them) because the infantry inside are immune to psychic power---in fact the Battle Bunker is completely immune to mind control, even when empty.
Should be noted that while cheap, a fully filled bunker does need abit of resources to get to its full potential. However even when filled with basic conscripts, the battle bunker is far more powerful and armoured than a sentry gun or a pillbox. These can make a rather powerful line of defense in a low power situation when built en masse, though they take signifcantly more room to build compared to base defenses.