The forces of Ladros City were on the losing side. The casualties kept growing every minute. More than ten thousand people have already died and it will continue to increase.
They were protecting the civilians while fighting powerful foes and that put them in a very disadvantaged position. So they created barriers on their battlefield to protect civilians. Maintaining these barriers while fighting the Seven Deadly Sins was a very hard thing to do. But what could they do? They couldn't just let those innocent people die in front of them.
If they didn't create a barrier on their battlefield. They could destroy this entire city in just one or two more hours. The aftershocks of their battle will wipe out this city on the map of the Hebrei Kingdom.