There are some magnificent Inca walls in the ruined ceremonial centre of Sacsayhuaman, on a hill north of Cusco.
The massive rocks weighing up to 130 tonnes are fitted together with absolute perfection. Three walls run parallel for over 360 metres and there are 21 bastions.
It has a double wall in a zigzag shape - some say this represents the teeth of a puma figure whose head the fort may have formed.
Sacsayhuaman also once had at least three fabulous huge towers, and a labyrinth of rooms large enough for a garrison of 5,000 Inca soldiers.
Archeologists estimate that tens of thousands of workers laboured on this massive structure for up to seven decades.