Guarded Arch 3D Optical Illusion

What happened here? Does this arch have three columns or two posts?

See Another Random Optical Illusion

Explanation

This optical illusion is a variation of the Impossible Fork Illusion.

This optical illusion is using the techniques of shading and perspective angles. The three columns are shaded, which makes us think they are round. Then, note that the guards are positioned in a darker, shaded area. That gives the impression of them being in a shadow. In this case, we assume it is the shadow of the columns. Also, one guard is smaller that the other one. We assume he is farther away.

If you cover up either the bottom of the columns or the top part of the drawing, it all looks correct. Because of the way the artist has connected all the parts, we see something that is not possible to create in reality.