The Inca Empire developed in a long strip that reached pretty much north to south along the western side of South America, the side that faces the Pacific Ocean. The geography was rugged - Andes mountains, the coastline deserts, and the Amazon jungle. The clever Inca found solutions for their geographic problems.
The Andes Mountains run north to south. The Andes are the backbone of South America. They run close to the west coast for the entire length of South America - 4,500 miles! In some places, they measure more than 400 miles wide. In others, they measure about 100 miles across. These are not flat miles. The width is a series of very high mountain peaks. Here and there in the Andes, there are active volcanoes. The Andes are rich in minerals including gold, silver, copper, tin, iron, emeralds, and more! The Andes Mountains are very high. They are higher than the Alps or the Rockies.
The Incas made their home high in the Andes mountains. To move about, 11,000 feet above sea level, the Inca built bridges between mountain peaks and over deep gorges. These bridges were made of sturdy vines. The Inca never invented the wheel, and the commoners were not allowed on the roads, so the only travel over these bridges were the animals that hauled food, warriors, the road runners (who carried messages using a relay system), the nobles, and government officials.
To stop invasion in any section of their empire, all they had to do was burn two bridge, one on each side of a deep gorge, which effectively captured their enemy in the middle. They did not even have to kill them. They could walk away and let the captured people freeze to death or be eaten by animals.
On one side of the Andes Mountains is the Amazon jungle. On the other side is the desert coastline. These both acted as natural barriers. The Inca did not try to establish cities in either the jungle or the desert, although they probably entered to harvest fruit and wood and seafood.