Player is too weak or too strong?

Mountain Mike

I tried to attack a city in Olynthus and it told me "the target player is either too strong or too weak for you to attack". I heard this is a "casual world" and I don't want to play another world like this, so when I am looking at the list of worlds, is there a way to tell from the list? I'm frustrated because it took me about 2 months to build up my city and then later, my army, then I try to make my first attack and I get this!

I put up a list of other worlds and screen captured it. Is there any indication that this world is "casual"?