Cannon are constructed using advanced metallurgy technology, allowing them to be smaller but more reliable than the elder bombards. Using solid metal (or explosive) shot they can bring down a castle from a great distance. In fact the cannon rendered castles obsolete, to be replaced by fortresses with low, angled walls which would deflect the shells up rather than try to withstand their force.
Cannons are efficient alliance troops that can help defend Town Centers. Their high damage can take out Generals and other strong units such as Heavy Cavalry. Cannons can also take a large amount of infantry if they are clumped together. Cannons are also ranged so they can shoot over walls against enemy troops without being attacked by melee units such as Heavy Infantry. This makes Cannons the number one donated war troop for defense.
Use the Angkor Wat to heal them as they attack enemy troops. That way, they can last longer as they are being attacked.