The Temple of the Sun is also known as Torreón which means fortified tower. It is made up of a semicircular wall and carved steps that fit seamlessly into the existing surface of a natural boulder, forming some sort of altar. Once used as a solar observatory, this unique complex is the only round building in Machu Picchu. At sunrise during the summer solstice, the sun's rays flood through the window and illuminate the tower with a precision only the Incas could have executed.