Custom levels created by users

I was wondering if there is a way to lets the users be able to create custom levels for my game. I know about assetsbundle, but I’d like to know how an user can create an external map… I think that he have to download the full Unity3d engine, am I right ?
Is there any other way to lets the users to be able to create their custom maps without using the whole unity engine ?

Thanks a lot for the help

Make an ingame editor. Save each map as a separate file. Then it should be easy for users to upload or share the files. That only works for PC though (for console you will probably have to let the players connect and send the files through the network)