Around the 5th century an impressive system of fortifications consisting of ditches and earthworks was built along the northern periphery of Tikal 's hinterland , joining up with the natural defenses provided by large areas of swampland lying to the east and west of the city . Additional fortifications were probably also built to the south . These defenses protected Tikal 's core population and agricultural resources , encircling an area of approximately 120 square kilometres ( 46 sq mi ) . Recent research suggests that the earthworks served as a water collection system rather than a defensive purpose .