I'm making the following assumptions about the guards going outside:

1) They close the door as they go out and they can reasonably get the rest of their gear in place. 
2) We have a similar system for knocking. A loud, solitary knock means "Let me in", while two knocks means "We're under attack, get out here and fight!" 

Also, how high are the crates? Would Goradh know what is in them? (I.e. if he decides to simply knock them over, would they contain supplies that would be destroyed?). His Athletics check is +10.