I don't know where they got them, maybe from earth turn undead? But some jackal lined the peerage ward's bridges with boulders. These boulders however are highly resistant if not immune to most forms of damage.
I suggest that they have massively less hardness or are unflagged from whatever lets them resist anything I throw at them via magic/physical attacks
I agree using boulders in that manner and them being indestructable is rediculous.
Just lift them out of the way.