It's not often we consider gravity in our everyday life, and maybe even less often when looking at a piece of art; in both contexts, heuristics are often used. The picture below provides a contradiction to natural thoughts and experiences, for certain things in the picture are not physically possible, such as the people walking on the upside down staircases.
Because of the way we are accustomed to see, it's likely that a viewer's eyes will follow the staircases in an attempt to find the plane in which the artwork was created. However, the artist did not use a standard single plane, but rather many different planes. Some aspects seem to be upright, others upside down, and some at perplexing angles that can confuse the viewer. The picture seems no more (nor less) rational rotated one quarter turn.
There are many perceptional tricks happening in this picture, but I will only cover one. We don't perceive linear perspective because there isn't a vanishing point in the picture due to the aforementioned multi-plane characteristic. If you rotate your head and view the picture from different angles, the stairs don't appear to converge as distance increases. Rather, there are three rough spots forming a triangle where the main staircases come together. This perception creates the illusion that certain aspects of the picture seem to defy gravitational laws.
I really enjoy these pictures, and was waiting for somebody to blog about this one in particular because it is a classic example. Just to tell a small story, last year for art class, my friend actually recreated this picture in his own way and instead of people, he drew dogs walking up and down and around all of the staircases. This picture and post brought me right back to that memory, but your trick that you say about viewing the picture from a different angle was surprising because I didn't know that it did that!