When it comes to environmental/easy levels, my general approach is very similar to Purple Ink's. I sometimes make a rough draft of the overall map/layout for the level before I start (by putting empty tiles in the level editor to mark which screens I'm planning to fill later), but I hardly ever plan ahead for each individual screen unless it's a screen with a specific purpose (puzzles, cutscenes...). I just select the next empty cell and start placing tiles in a way that looks good/interesting to me. However, when there are challenges involved, I usually start by trying out different obstacles/puzzles (using placeholder walls) until I find a combination that works, then I start remodeling the screen to make it look nice, while also trying to not break the drafted challenge nor make the enemies/obstacles too hard to see.
I too start most of my screens by placing the solid tiles (layer 3). I first make a draft using only inner wall tiles with no corners. I'm not a fan of using a single set of walls to make one big contiguous block for the floor/ceiling, so I always try to mix at least two different sets of walls instead. Once I'm happy with the layout, I start replacing the filler tiles with borders and inner corners where appropriate. I may also add some irregular borders here and there to make the screen look a bit more detailed and "3D" (for a lack of a better term), as canteven has already described. Also, if I decide that the current screen should have running water and/or pools, I add those as well before moving on to the background tiles. Generally, I place waterfalls in layer 4 and water objects in layer 5, unless I want the waterfall on the foreground.
At this point, I get started on the background. I usually start with a simple gradient as a base and customize it to add some far scenery (for exteriors) or a texture/pattern (interiors), then I start adding elements (grass, trees, pillars...) until the screen feels distinct enough and stops looking empty/monotonous to me. If it's an interior, I may decide to either not use a custom gradient at all (using layer-0 tiles over a black gradient instead) or make one or two custom, more detailed gradients with all the elements I need (which is what I did in
Curse of the Catacombs). This is also when I start adding shading tiles (which I use in almost all my levels) where appropriate. Adding shadows below ceilings and floating platforms is simple enough, but placing them between multiple layers of solid walls requires more work, since this involves moving the original wall tile to a non-solid layer and then placing the shadow in layer 3. I make this a bit easier on me by adding an invisible square collision mask to my shading tiles directly in the tilesets, so I don't need to add invisible objects later to cover the "holes". I still mess up from time to time when doing this, though
Lastly, I proceed to add some extra decorative objects and/or harmless creatures if I deem it appropriate. I generally don't add more than 1-2 creatures per screen... unless these are fireflies/specks, in which case I go nuts with them
There's also the matter of using tints and attachments in KS+. When I'm designing a dark area (a cave or a nighttime exterior), I sometimes use a SUB tint set to a very dark shade of yellow. This darkens the screen a bit and gives it a slight bluish tint, which increases the contrast between dark backgrounds and flames/lamps. On the other hand, if the screen has multiple glowing orbs or other bright sources of light, I may use an ADD tint (set to a very dark shade of gray or whatever color matches the overall tone of the room) to enhance the shining effect. I also like to use dark attachments in segments that I think should feel more dark/creepy/claustrophobic than the rest, although I prefer to use custom black gradients with a rather big visible area than the default one (especially in screens that have traps or enemies, since this can cause a lot of cheap deaths).