The following units can be upgraded to Cannon:
Cannons can bombard enemy city fortifications. When attacking another unit, they cause collateral damage to other enemy units sharing the target's space.
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.