I think kettlefish needs to weigh in, but based on my look at the German rules, it appears that the word "effect" isn't even in there. The German "Eine burg kann auch mit Wagen gebaut werden" translates as "A castle can also be built with a wagon."
Thus, I think they are simply saying that a wagon can occupy a castle like a normal follower. I don't believe this is trying to change the turn order. It looks like "wagon effect" just wasn't a very good translation.