There are several ways to accomplish this but I have been using this method for a while and seems to be the easiest one; it is very fast too. Just go to the command prompt and navigate to the root of the folder you want to work on and use:
ROBOCOPY folder1 folder1 /S /MOVE
As you can see, we use the same name of the folder as the destination folder too. What this command actually does is move all the content to a folder with the same name but it skips the empty folders (remove them).