After the decepticon sensors clear the explosion caused by Enkidu and Gilgamesh fighting, they discover there are four power sources within the city. Each of them is a nuclear power plant, placed in separate underground bunkers to minimize the risk of damage. This is the only reason why, even after the explosion, they remain relatively intact. However, further fighting could easily endanger the borders of the reactors, making them dangerous to any nearby and even causing risk of a nuclear meltdown. Worse yet, with the kind of backlash the weapons in this battle can have, and the area of effect, the Decepticons could easily get affected simply by being this close to the scene of the battle, making this ground quite dangerous as long as the fight rages on.
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Stein turns, as he witnesses the massive, city-sized explosion in the distance. His soular sense told him that it was Gaia, fighting two unknown but highly powerful entities. The battle would be more than enough to distract the large robots for the moment. "That explosion is from where I saw him last. It was some kind of giant robot, called himself 'megatron' I believe." Stein had also, of course, detected Megatron's presence on the scene, and he fitted the role of villain perfectly. Once he managed to weaken both parties in battle, he'd finish of whatever side won. He might have to create a distraction of some sort for the fire-wielding demon boy, but other than that the plan had no Obvious flaws. Perhaps the explosive nature of that area could be considered one, but Stein knew he could teleport out before any severe damage reached him, a luxury that the two giant robots lacked.