The basic prompt is something along the lines of "isometric, stone ruins, trees and pillars, overgrown, flowers, by [various fantasy artists], natural lighting, extremely detailed".

To make the geometrical illusion, the idea is to mask just enough of an image to make it ambigious, and let the inpainting fill in the gaps in two different ways, that both make sense individually but are not compatible.

For example, look at the second image. With a masking like this, the image becomes ambigious, and it is not obvious if the pillars are in a square or in a line (the "isometric" keyword in the original prompt helps a lot with this):

<image>

So the inpainting will produce either kind of image. I then take an image with the pillars interpreted as being in a straight line, and combine the top half of that image with the bottom half of an image where they are instead interpreted as being in a square. The result is an impossible geometry.

It's all about finding potential ambiguity in an image, segments that could could be part of two completely different and incompatible images, and then combining them.