Chokepoint
The chokepoint in the tunnel was narrow to the point that only a hundred espers would be able to stand side by side. The guards had thrown together crude fortifications from a mixture of steel and manullium. They had also mounted steam cannons atop the fortifications, turning the blockade into a dead zone for any prisoners that got too close.
"Steam Tanks to the front!" Tai yelled as he rose his arm in the air and waved it downwards.
Ten steam tanks moved forward, barely able to maintain their formation in the narrow tunnel. When they got close enough, the guards fired their cannons. Explosions echoed everywhere as the tanks were engulfed in black corrosive fire that vanished just as quickly as it came. When the scene cleared, the tanks were completely destroyed. The remains even blocked the way forward, preventing more tanks from moving forward to replace them.