The Aztecs built causeways by using a foundation of wooden stakes, rocks and clay covered with a puzzle-like layer of fitted wood pieces. The upper layer provided a firm foundation and made it possible for the Aztecs to quickly disassemble the causeways if the city came under attack.
The Aztecs became adept at building new land over water because their main city, Tenochtitlan, was built on swampland in Lake Texcoco. As the population grew and the city needed to expand, Aztec engineers developed techniques for adding to the original two islands that comprised the city. They drove thick wooden stakes into the bed of the lake and then laid a layer of heavy stones to form the foundation. From there, they dredged up mud and clay from the bed and piled it over the stones. The tall stakes kept it from spreading out again.
There were three main causeways in Tenochtitlan. They were critical to Aztec life because they allowed people to walk between the islands and from the islands to the shore. They were not the only major Aztec engineering feats. Lake Texcoco's waters were salty and unsuitable for drinking, so the Aztecs also engineered aqueducts to provide water for drinking, cooking and cleaning. They had an elaborate system of canals that they also used for navigation, earning them comparisons to Venice, Italy.