Trading Towers

Notice the diagonal lines passing through the middle of the figure- when you see the tall thin towers, then these diagonals seem to bend once, right at the center of the figure. This breaks the rule that straight lines in an image must be seen as straight lines in 3D. When you see the short fat towers, then the vertical and horizontal lines also seem to bend in the center, again breaking the rule. Your visual intelligence dislikes breaking this rule: its processes of construction automatically search for alternatives that don't break the rule, which may explain the instability of the figure. Notice how briefly you see each interpretation before flipping to the other.