In the making of roads, bridges, aqueducts, and irrigation ditches they showed a remarkable knowledge of engineering. At the time of the Spanish Conquest the Incas paved roads ran for thousands of miles through the Central Andes from Quito, the capital of Ecuador, all the way to Argentina and Chile, as well as from Pacific Cost over the mountains to the warm valleys of the eastern Andes. Since they had no wheeled vehicles it was not necessary for the surface of their roads to be leveled. Where the road had to be taken over a steep hillside, stone stairways were constructed. Where the road had to pass a small precipice, tunnels large enough to permit the passage of a loaded beast of burden, whether man or llama, were cut out of the solid rock.

Over these roads trained runners, operating in relays, carried messages with extraordinary dispatch from the capital of the empire to distant magistrates. It is said that fresh fish caught in the Pacific Ocean were brought over the mountains by the special messengers of the Inca Emperor and reached his table in excellent condition. Post houses were provided at convenient intervals so that a runner would be picked up and carried forward with the least possible delay. Furthermore, the runners were permitted to chew coca leaves to deaden fatigue.

The Incas had never acquired the art of writing, but they had developed an elaborate system of knotted cords called quipus. These were made of the wool of the alpaca or the llama, dyed in various colors, the significance of which was known to the magistrates. The cords were knotted in such a way to represent the decimal system and were fastened at close intervals along the principal strand of the quipus. Thus an important message relating to the progress of crops, the amount of taxes collected, or the advance of an enemy could be speedily sent by the trained runners along the post roads.

Caravans of llamas carrying supplies could proceed safely, if slowly, over the most mountainous country. Tambos, or rest houses, as well as storehouses, were built wherever it was likely that those who traveled on the Inca's business - and there were no other travelers - would need to find suitable accommodations and supplies. The store-houses were large enough to provide for companies of soldiers as well as llama drivers.

Governor of the bridges of this kingdom, chaka suyuyuq.
Illustrations from 1615 by The "Indian Chronicler" Felipe Guaman Poma de Ayala

The roads were carried across rivers on suspension bridges made by braiding together countless strands of lianas, the ropelike vines found frequently in the jungles of the Amazon Basin. Using huge cables of remarkable thickness, the Inca engineers were able to construct bridges two or three hundred feet in length whenever it was necessary. These bridges, of course, sagged in the middle, swayed in the wind, and were not at all pleasant to use. Furthermore, they could be destroyed easily, but the death penalty anyone found guilty of such an act. Had they not been so highly regarded, or had the Incas had the foresight to destroy them when Pizarro and his conquistadors started to enter the Central Andes, the conquest of Peru would have been extremely difficult, if not well-nigh impossible.

Source:

‘Lost City of the Incas, The Story of Machu Picchu and its Builders’ by Hiram Bingham
The American explorer who found the ruins of Machu Picchu in 1911.

Hiram Bingham at Machu Picchu
The inspiration for Indiana Jones?