The adjectival clause here is 'where the game was being played'. Because an adjectival sentence is a subordinate sentence, and this is the only subordinate clause, plus it modifies a noun, Cleveland, so it has to be an adjectival clause.
The poster is a persuasion because it talks about undermining war effort, which you would not want to do, so its is persuading you to do you part in silence. that means not giving info away
