Both the Maya and Aztec civilizations settled amid the dense and luscious forests of Mesoamerica, now modern day Mexico and central America. The Maya civilization in particular is especially well known for their temples and architecture, as can be seen here.
Although the Mayans did eventually die out around 900 AD., likely due to famine caused be a drought, they still managed to leave their mark via their hieroglyphic writing system, numerical symbols, violent religious practices, and their architecture.
Based off of the island city of Tenochtitlan, also in the Mesoamerica reason, the Aztec Empire is now perhaps regarded as one of the most famous American civilizations. Much life the aforementioned Mayans, the Aztecs also built several pyramids and religious structures to honor and praise their gods.
It is also to be noted that the Aztecs were a notoriously violent civilization, both in war and in religion. Their empire was built almost solely off of conquest, and the emperor's main job was not to govern the people, but to lead them in war. However, in 1521, the empire was eventually taken over and dismantled by Spanish conquistadors.
Meanwhile, on the mountainous coast of modern day Peru, Chile, and several other South American nations, the Inca people dominated the Andes. The Inca also lived under a supreme ruler, an emperor, of whom had complete and total control over the entire empire. The Incas can also potentially be thought of as "paranoid" people, for the cause of their mountainous location was almost purely for defensive purposes.
They were also perhaps very advanced, as well, for, unlike the Europeans at the time, they were able to perform successful surgeries on unconscious patients. Unfortunately, several factors lead to the fall of the empire, who's legacy remains mostly in their few remaining cities.