Welcome to Chief Delphi and FIRST!
To answer your questions in order:
Yes, the boulders are reusable, however, you opponents reenter them into the field after you score.
You are allowed to carry one boulder at a time, and tbere are no exceptions to this rule.
You are allowed to block a team from shooting into the goal, however you are not allowed to stop a team from getting to the tower to scale the castle.
If you have any more questions, don't hesitate to ask!
