Context : OnRoads (some stuff refactored there from this page)
The Incas, like the Romans, built a vast empire. (3000 km in length). But it´s not obvious they had a special advantage in weaponry or military strategy. But one advantage was their road network. The GreatWallOfChina is also a road.
Also interesting. Unlike most large scale civilizations, the incas didn't have writing. However, they could represent certain information in khipu or specially knotted string which are started being decoded.