Medieval ranged infantry Unit. Only the Chinese may build it. This Unit may fire twice in one turn, unlike the Crossbowman which it replaces.
The Chu-Ko-Nu is the Chinese unique unit, replacing the Crossbowman. The Chu-Ko-Nu is slightly less powerful than the Crossbowman, but it can attack twice during a turn, giving it a hefty offensive punch. This unit is particularly deadly when garrisoned in a city that is under attack.
The Chu-Ko-Nu is a rather nifty repeating crossbow invented in China at the end of the second century AD. The machine looks something like a standard European crossbow with a box bolted to its top, just above the string. The box was filled with crossbow bolts. In operation, the soldier would pull a lever to cock the string and then push the lever back, causing a bolt to drop out of the box and into place to be fired. This gave the Chu-Ko-Nu a faster rate of fire than a standard European crossbow. However, the Chu-Ko-Nu fired a lighter bolt and had a shorter range than a European machine. Which was the superior weapon would depend upon circumstances - the European model might prevail in open terrain against heavily armored opponents, while the Chu-Ko-Nu would probably be better at short-range against large numbers of lighter-armored attackers.