This topic is very similar to "how do you design your levels" but a bit more specific. I was thinking about this whilst working on my current project. So, how do you design your screens?
For me, I rarely plan ahead for any one screen's layout. I just start making stuff. For instance, in Advena I knew I wanted a vertical area with Hypericum so I knew the screens would connect top to bottom. But I had no ideas for what the middle of the screens would be like. If it's an area that I don't have a concept for I just make screens and put them together until I like how it flows together. Once a screen is done I immediately test it. If I don't like how it looks or feels I'll rework it or delete it and start over.
With Advena I knew what I wanted screens to look like so I made the tilesets to match what I had in mind. With my current project I find myself adding as few as one tile to make a screen look exactly how I want. I've also expanded from having one tileset per area. Not sure why I thought constraining screens in an area to one tileset was a good idea. It's rare that I'll have a background planned for a screen. I tend to just randomly pick a background that matches the tileset and fits with the scene I have in mind. If I can't find one, I try making one.
In a similar vein, I have the scenery in mind so I know what kind of music I'd like to match those screens. I usually don't include music on transitional screens (screens between two areas) or other connective type places. I feel like the music should match every screen for a given area. I actually spend a lot of time searching for music to fit the screens. Ambiance is another thing. Some screens should have silence but that can be boring if overdone. So I try to include some kind of ambiance in every screen. That's part of the reason why I include water or waterfalls or both in a lot of screens. It just makes sense for there to be water in, say, a forest-y area.
Another thing I like to do with nearly every screen is include some kind of animal. That way the level feels alive. This could be bad, however, because there could be too much and it could be distraction from the experience instead of enhancing it. So I tend to limit how many creatures I include: typically two at max. And on the subject of creatures, I try to only use a certain kind in any one area of screens. When I first started Advena I didn't do this. But as I continued I started to favour only one kind of creature per area. To me it helps the screens feel unique beyond music and graphics. In my current project, different areas that use the same tilesets will have unique creatures to help distinguish them.
Enemies are another thing. I now try to design levels that anyone, or nearly anyone, can play and enjoy (hence why the challenges were optional in Advena). Depending on the kind of enemy I'll only put as many as two on one screen. Some enemies I won't use at all, like the green chomper. I hate those guys so why would I use them? Another enemy I refuse to use is 4:14 because I still don't understand how it works. I also try to give an easy way to avoid some enemies. Though, where would the fun be if every enemy was superfluous?
When I'm making a screen I always start with layer 3. Sometimes I have an idea in mind of how I want the shape to be or where I want the screen to go. Often times not. I try to make the ground and ceiling interesting without too much flatness or repetition (thanks Knytt Syndromes). Unless I'm restricting movements based on power-ups or similar I try to make everything readily accessible. That is, it's not a max jump height to grab a ledge and climb up or no corner jumps required unless it's for a secret or something. After layer 3 I fill in the background starting with 0 then 1 then 2. After that's done I put the extra stuff like creatures and other objects. If possible I always put these on layer 4 for consistency's sake. Except water, which always goes on layer 6.
I feel like I rambled a bit there because this was a bit longer than I was intending. Anyway, how do you approach making screens?