The Andes Mountains stretched 4500 miles. It is higher than the Alps and the Rockies. This made it difficult for enemies to invade, yet cut off trade.
The Andes Mountains has active volcano with a lot of rare earth minerals. The Inca people also built bridges that could be burnt down to isolate the invaders. The Incan people did not even have to kill them. As they died from animals or froze to death.
The Incans were very isolated. Trade was cut off from all sides, The amazon jungle to the east. The coastal desert and the Pacific ocean to the west.