The defensive fortification of Benin City, the capital, consisted of ramparts and moats, called iya, enclosing a 4000 square kilometer area (2485.5 miles) of community lands. In total, the Benin wall system encompasses over 10,000 kilometres of earth boundaries. Patrick Darling, an archaeologist, estimates that the complex was built between 800 and 1000 AD up to the late fifteenth century (Keys 1994: 16). Advantageously situated, the moats were dug in such a manner that earthen banks provided outer walls that complemented deep ditches. According to Graham Connah, the ditch formed an integral part of the intended barrier but was also a quarry for the material to construct the wall or bank (Keys 1994: 594). The ramparts range in size from shallow traces to the immense 20-meter-high rampart (66 feet) around Benin City (Wesler 1998: 144). The Guinness Book of World Records describes the walls of Benin City as the world's second largest man-made structure after China's Great Wall), in terms of length, and the series of earthen ramparts as the most extensive earthwork in the world.