The game uses the 'field cannon' as a Ranged unit. The problem is, once a gun carriage or mount with trails and trunnions on the cannon itself was introduced (1460 - 1490 CE) , which allowed the cannon to be easily moved and aimed, ALL cannon were technically identical, and the only difference between Siege Cannon and Field Cannon was the size: anything firing a cannonball weighing more than about 12 pounds was a siege piece, anything 12 pounds or smaller was a 'field' piece for most of the 17th, 18th, and early 19th centuries.
In a Perfect Civ Game, I'd like to see the Field Cannon remain as a Ranged Unit, because it was in fact the primary Ranged (over 200 meters) weaponry of all armies from the late Renaissance to the end of the 19th century (upgraded in the late Industrial Era with rifled barrels, breechloading and explosive shells, superseded after 1900 CE by indirect fire 'modern' Artillery) but add the Howitzer as a Industrial Era Siege Unit - it was developed by the Swedes at the end of the 17th century (either very beginning of the Industrial or very end of the Renaissance Era) specifically to attack cities by lobbing explosive shells over the walls.
Technically, there are no dedicated 'siege' weapons in the Modern Era and later: regular artillery and bombers can do all the damage necessary to storm a city without having to dedicate resources to a siege-only set of weapons. The super-heavy guns developed by the Germans in WWII: Dora and Gustaf railroad monsters, were Mistakes and an incredible waste of scarce resources, since they each required the equivalent of a regiment of men to operate and were less capable than a single squadron of heavy bombers in range and weight of explosives delivered.
