User blog comment:3primetime3/What's up with the "Funnest level in the Game Poll?"/@comment-27403314-20140713030833/@comment-27403314-20140714121029

There isn't really a fixed meaning for fun in CCS. The meaning of 'Fun' differs in everyone. Some say a level is fun when you complete it quickly and easily, some disagree too.

Difficulty is more fixed compared to it. A level is probably hard when you complete it in a lot of tries or complete a level in a close call.