Koricancha

This is the most important Inca site in Cusco. It was across the street from the hotel, so I did see it, at least from the outside.

It was a temple to Inti, the Inca sun god, and the walls were covered with sheets of gold (gold that was eventually stripped to pay ransom for the life of Atahualpa, though of course the Spaniards killed him anyway.) After the fall of the Incas, the Spanish demolished the temple and used the stones to build a church on the site. You can still see some of that amazing Incan masonry in the foundations though.