Inca civilization was structured as

as an empire on top of mountains so no intruders could get in.

A large empire with one ruler, is the right answer.

Explanation:

The Inca Empire was the most extensive empire in the pre-Columbian U. S. Its administrative and political construction is acknowledged by most historians to have been the most advanced in the Americas before the arrival of Columbus. The organisational, administrative and aggressive focus of the empire was found in the city of Cusco. However, the Inca civilization was structured as a large empire with one ruler.