Eh... There isn't any explanation about why putting slimes in holes is good?
No telling people it is to protect their cores so that when goblins thrash about they don't smash them?
Why leave the sides open? I get you are on a raised cliff, but you literally just build up a mound of dirt to shoot arrows from. Do that to the sides as well so the goblins cant actually avoid the acid.
@Milanin
As I understood it, it was so they could grap the goblins and make them stay inside the acid.
@JavelinJoe
Might be to avoid the goblins just going another way, with the sides open it still feels like there is a way to survive running this way, if the goblins see they have to swim an acid pit before then having to fight around 50 people, they might just jump of the cliff for a better surviving chance.