Castle drops on Arena

So I've done some searching and am having a hard time finding any discussion on castle dropping on arena. Its a very common strategy and I'd like to know more about it. Some questions: How many vills do I need on stone from dark to castle in order to have enough stone for an aggressive arena castle drop? What units do you like to send in with a castle? What civs have good strategies that can work around castle dropping on arena? How do you respond when you see a castle start to go up outside your walls?