User blog comment:Zaarqawi/Why players from YGGDRASIL have been transported to the New World at various periods of time./@comment-27008999-20151110194334/@comment-27008999-20151112003510

No, Ainz just wondered what the surroundings were and Sebas said there was grasslands. Grasslands means Nazarick had been moved to an YGGDRASIL world that were either Middle-Earth, Alfheim or Vanaheim. So Ainz knew that the 8GKs castle were in one of those worlds because a floating castle would stand out if he ever visited one of those worlds.