Because an obelisk is a square column with a pyramid at the cap, from a top down view, it only looks like a pyramid. Often when I create map objects with angles hidden due to being top down, I imply the shape with the shadow, rather than the object. I am working on an Egyptian based city design, so I too am creating obelisks for my map, I create a directed shadow (in the same direction as any drop shadows used in map) that depicts the obelisk form, unlike the top down view does.

Like this...

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