Thanks Falconius

The landscape is a procedural terrain that I created in Vue, and textured with my own grass textures and a couple of rock textures from textures.com. I rendered it using a camera with the focal length set to 500mm (there aren't any true orthographic camera settings in Vue, so you just have to make it as close to isometric perspective as you can with the focal length). The reason I used Vue is because I get totally tangled up trying to texture paint models in Blender, even though Blender does actually have a proper orthographic camera setting.

Next I imported the render into that grid file I uploaded before, and trimmed the image to fit the equatorial plane. I then used GIMPs Filters/Map/Map object filter to convert a flat sheet of rock and a flat sheet of soil to spheres, and trimmed the spheres to create the fill beneath the landscape.

Then it was just a case of importing everything into the main file and scaling it down to fit the chosen bubble.

I'm going to draw each portal in a separate file and import them into the main drawing like I have done with this one, and if I don't like the arrangement I will switch them around.

This is only the draft just yet. I've a very long way to go...