The OldCastle is a world originally generated as an old world type. The old world type used to be the only available world format for Minecraft PE way back in time. Even though that's a slight downside it's still well worth checking out since it features one ginormous fantasy castle with tons of majestic rooms to explore.
Creator: Faith_shupee, Twitter Account
And if you really want to be able to use it in your survival world you could always try to export it to an infinite world within the game. Simply edit the world and press Make Infinite. Parts of the map will look messed up but that could be fixed by terraforming.