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Boulders from rubble will not do any damage, but those from bedrock or catapults will damage and possibly kill any players they touch. A knight will get stunned and receive damage from a boulder even if shielded. However, the damage may be reduced by shielding, and it will stop the boulder(s), protecting nearby team mates. Typically, after a boulder is launched, it will bounce several times off of the ground or built structures before rolling to a stop. The boulder will do damage to all blocks except bedrock, and will instantly destroy ladders. A boulder shot from a catapult will not hurt the allies of the one who shot it, passing through them instead.
Each boulder can be picked up from the ground when a builder walks over it, yielding three stone.