One of the reasons the citadel has and will last for so long is its harmony with the flowing energies. The builders of Machu Picchu not only used their intelligence, but also their hearts in the creation of the structure.
They emulated, Pachamama (Mother Earth). Using physical building blocks but choosing each action through love, beauty and harmony, thus creating a portal conducive of magic. This is the energy everyone feels so strongly when they visit.
All windows were specifically located to align and point out the solstices and equinoxes of the year.