You draw the levels; the editor draws the ground between them. A cog on the 3D view decides how that ground is shaped and coloured.
Where the settings are
Click the cog in the 3D view's button row. It holds the 3D rendering options and, under them, the terrain surface settings.
Terrain surface on or off
Terrain surface is the same switch as Terrain in the top bar. On, the areas fuse into one landscape. Off, they stand as separate solid blocks and the settings below do nothing — a useful way to see each level on its own.
Rounding
Rounding turns the rim of every plateau and the wall of every pit into a bank about twice this wide. Small values give crisp kerbs; larger ones give the rolled edge of a lawn meeting a slope. It applies to the ground only — objects and structures keep their shapes. An area with Hard edges ticked ignores it and steps straight up.
Mesh detail
Mesh detail is how finely the grade between areas is resolved. The edges of your areas are exact whatever this is; the setting only changes how smoothly the ground in between curves, and how much work the browser does. Turn it up for a final look, down while sketching on a slow machine.
The filler
The filler is the open ground and slopes woven between the shapes. Give it a colour, or a texture — Grass for a garden, Gravel for a yard. Textures show in 3D; the plan shows the colour.
Shadows and quality
The same cog has Shadows and a Quality switch. Low is fastest; high is sharpest with crisper shadows. Neither changes the model — only how it is drawn on screen.
Next
Read a cross-section — see the ground profile you have made.