The Temple of the Sun is a rounded, tapering masterpiece of stonework, probably the finest in Machu Picchu.  These stones are carved and smoothed to fit together seamlessly, and note how they are rounded and tapered.  The window is aligned for the June (winter) solstice, illuminating the stone at the center of the temple.  So important is this historical structure that it is roped off and you can't go inside the temple.