While this is not a level design book, I've to say that designing a level and design in general is not about tools, techniques, and technology. In fact, it has nothing to do at all with techniques or engines. It is an artistic and creative thing.
But there are always major points that you have to keep in mind while designing a level; these points depend on the game itself, its genre, its story, and lots of other factors. But for Bellz, I've found the following points come in handy to the game:
The level has to tell a story: I need the player to tell the story of the level through it. I don't want to show the player a movie describing what happened here before he came to the level, and I don't want to display text to tell what happened at this place. But I want my level itself, and the positioning and selection of assets, to tell the story of the place on its own.
The level needs to look eye-catching: Having a fantastic game is not only a result of using good and polished...