The Incas ran a highly organised empire high in the inhospitable South American Andes from the 12th to 15th centuries. Much of what remains of the old Inca civilisation is now found in Peru.
The great city of Cusco, the Inca capital, was divided into four sectors representing the four corners of the Incan world.
The city was laid out in the shape of a puma with the magnificent fortress of Sacsahuaman forming the head, its staggered walls representing the teeth. Stone sundials and pillars were set up to the east and west of the city to record solstices, important in Inca life.
A surprising number of Inca relics have survived. So well constructed were they that while earthquakes damaged many of the Spanish buildings later built on top of ruins, the Inca substructures remained intact.
The Incas built immense stone fortifications for their cities, often carving them into the sides of mountains. Huge stones for the walls had to be transported from quarries by teams of labourers.
The fortress at Sacsahuaman is one of the most impressive. The site of many fierce battles, it still overlooks Cusco. Its footprint is 900m long and fortification is made from stones weighing up to 101t. It originally had three turrets connected to the Incas' palace via underground passages.
All stone for the city would have been quarried and transported over the mountainous terrain. Heavy granite rocks were cut and rounded into interlocking shapes using stone or bronze tools. It is thought rounded shapes were favoured as being less prone to erosion. Boulders in the fortress wall were so tightly packed that even today it is impossible to insert a knife between them. A lot of the smaller rocks have since been carried away and used for house building by local people.
The Incas' advanced masonry skills enabled them to blend fortresses and posting stations into rocky outcrops, hiding them from potential attackers.
The achievements of Inca engineers are all the more remarkable because they built their castles, road networks and water supply systems before they had invented the wheel.
They had also still to invent arches, relying instead on using instead massive lintels in gateways and doors. This gave their buildings a chunky appearance.