In 1131, the Chinese writer Zhang Yi noted the importance of employing a navy to fight the Jin, writing that China had to regard the sea and the river as her Great Wall, and use warships as its greatest watchtowers. Although navies had been used in China since the ancient Spring and Autumn period (722–481 BC), China's first permanent standing navy was established by the Southern Song in 1132. The Jurchen launched an invasion against the Southern Song along the length of the Yangtze River, which resulted in two crucial Song victories at the Battle of Caishi and Battle of Tangdao in 1161. The Jin navy was defeated by the Song's standing navy, which employed trebuchets on their ships' top deckhouses to launch gunpowder bombs.