The Inca had advanced knowledge of engineering. What did the Inca build to keep their empire connected?

The Inca built a good and extensive network of roads across their empire.

Explanation:

The Inca empire was the greatest and largest empire in South America, at least from what is known so far. The empire had very good mechanism when it came to ruling, assimilation, but also acceptance and unity among the people. In order to keep the empire well connected and to set their policies in place the Incas built roads.

The roads that the Incas built were of very high quality. Unlike in most of the world, these roads were made in areas that were very tough and harsh, mostly on high mountains and very steep places. This South American civilization though was highly specialized for living in those kind of places and their architecture reflected that, with lot of roads still existing even nowadays and being in very good condition despite not being maintained for centuries.

c. troops marched forward in close formation against newer, more accurate weapons