The Aztec skill at engineering was demonstrated after about 1300 by the establishment of Tenochtitlan or the precursor to Mexico City built on an island on the west side of Lake Texcoco. Causeways were constructed to the island with bridges that could be opened to prevent attack by enemies. Also, aqueducts were built to bring freshwater to the city and levees were built to separate brackish water from fresh, spring water.