The Aztecs didn't have "natural barriers" in the same way we might think of them, like mountains or oceans. They lived in the Valley of Mexico, a fertile but relatively flat area surrounded by:

* Lake Texcoco: This large, shallow lake acted as a natural moat, making the area relatively isolated from invaders.

* Mountains: While not impassable, the surrounding mountains (Sierra Nevada and the Sierra Madre Oriental) provided some natural defense, making it harder for armies to approach from certain directions.

* Swamps and marshes: The areas around the lakes and rivers were often swampy and difficult to traverse, making it challenging for invaders to reach the heart of Aztec territory.

However, it's important to note that:

* These barriers weren't absolute: They were easily traversed with enough effort, and the Aztecs themselves frequently used these routes for trade and communication.

* Military strategy was crucial: The Aztecs relied heavily on their own military prowess, fortifications, and alliances to defend their territory.

* They weren't isolated: While these features made the Valley of Mexico harder to invade, they didn't prevent contact with other civilizations. The Aztecs were active traders and had extensive networks throughout Mesoamerica.

So, while the natural features of the Valley of Mexico did offer some protection, they weren't the primary reason for the Aztecs' success. Their strength came from their sophisticated military, their political organization, and their control over resources and trade.