Flesh Golems
*BOOM*
The explosion at the city gate not only shook the people that are retreating but also the people inside the city. They didn't expect that after preparing so well, someone would still manage to blast open their entrance.
The losses are incredible.
But the main purpose of this explosion is to show Sam's firepower and resourcefulness.
The formation he gave Kiran is that of a controlled explosion formation and they used the fire elemental energy cells.
It is not as lethal and destructive as the controlled formation he used against the first branch leader, rather it is a very limited and most basic controlled explosion he created.