Who began the construction of a wall in ancient China, to keep out invaders from the north, known as the Great wall of China?

The Great wall is a mighty monument to the building skill of ancient China. This wall, built of stone and earth to a height of 6 metres and extending over 2400 kilometres of hills and valleys, links the east coast with the mountains of the west right across northern China. The wall was wide enough for a carriage to go through.At intervals of every few hundred metres were located watch towers for warriors. Such a large study construction must have taken decades and an army of labourers to complete it.