Can I use my single player world as a Minecraft Realms world? I have a castle map on there and want to use it with my friends.
Yes, you can upload your own maps to realms.
From the FAQ
Yes, with the release of 1.7.10 it is possible to upload your own world to your realm.
Yes! As of 1.7.10, you can upload your single player world to Minecraft Realms. The non-obvious bit is that you need to create a world in Minecraft Realms first, then overwrite it with your single-player world.
First create a worldin Minecraft Realms:
Once you have a world in Minecraft Realms, here is how to overwrite it with your single-player world: