The list of units
The Cannon is a more advanced artillery unit, capable of bombarding adjacent targets to reduce them before other units attack outright. Cannons have no attack or defense value, so they should be protected by an escort. Further, they are wheeled units, so they cannot enter mountains or jungles unless following a road.
A city must have both iron and saltpeter in its Strategic Resource box to build cannon.
Closely following the invention of gunpowder, the development of the cannon caused a revolution in siege warfare. Invented by a German monk in the 14th century, early cannons used gunpowder charges to fire rocks or metal balls. The cannon forced a redesign of most fortifications because the straight, high walls surrounding most cities could be easily destroyed by direct fire. This was the fate of the walls of Constantinople, which had held against countless sieges for over a thousand years. The cannon quickly assumed an important role on the battlefield.