Why were the inca considered a highly sophisticated civilization and empire? what was their most valuable achievement or advancement and why?

Inca Empire was successful because it had a powerful army and they were able to make effective weapons and tools out of their environment.  

Explanation:

Inca Empire comprised of various ethnic groups with diversified languages and culture being followed in the empire. The empire extended from Equador to Columbia and it encompassed Peru, Chile, Bolivia and Brazil. They had an organized officialdom to ruin the whole empire. Road system was perfect, taxes were fair and genuine and there was no shortage of food supply. They built huge archeological sites called Manchu Pichu. They practiced astronomy and they built many godowns and storehouses for the ambassadors to stay and carry the communication to respective places.

Network of shrines which was engineered in an aesthetic manner echoes the success of Inca Empire.  They accounted the information with the threads of ilamas and alpacas which were made in the form of knots. It signified the numeric value. Inca Empire is rugged and undulating land which made irrigation possible but with great difficulty.  

basic principles: federalism, separation of powers, checks and balances, judicial review, representative democracy, limited government, rule of law, individual rights and responsibilities, consent of the governed, majority rule, popular sovereignty, equal protection.