Refinements in cannon-casting techniques bring along the terrifying field artillery! With 20 more Combat Strength and Ranged Strength than its predecessor, the Field Cannon is a big step up from the Crossbowman. Its far-reaching cannonballs can devastate Knights, do serious damage to Musketmen and Pikes and Shots, and are reasonably effective against Cavalry and the defenses of most Industrial-era cities. Take advantage of their range and power to earn them a few Promotions before upgrading them.
However, due to the ever-changing landscape of war, the Field Cannon is now relegated back to a defensive role more than an offensive one. The introduction of Bombards to replace the outdated Catapults means that ranged units like Field Cannons are not required to aid in city siege like Crossbowmen are, which is the single reason why you see Crossbowmen everywhere in the Medieval Era but not necessarily Field Cannons in the Industrial Era. (Read more here.) Of course, you can still upgrade all the Crossbowmen you have into Field Cannons and bring them with you on conquests to shoot down units, but if you have to train a Field Cannon from scratch, it's probably because you need to garrison Districts for defensive purposes, or because you lack Niter to train Bombards for city siege. For offensive purposes, there is almost always a better way to spend your 330 Production, provided that you have the necessary strategic resources.
Field cannon – the earliest form of mobile gunpowder artillery – dates back to the wars of Gustavus Adolphus, a bastardization of the Ottoman hand cannon and the heavy siege artillery used throughout Europe. The Swedish king needed increased mobility and devastation from his guns if he was going to conquer the north, and so he developed 4- and 9-pounder demi-culverins and a 12-pounder cannon (the traditional method of distinguishing type of cannon is by weight of shot); each gun was able to be serviced by three men and hauled by two horses. He also organized them into batteries, rather than scatter them about through the ranks. And the Swedes pioneered several types of specialized ammunition, including canister for blasting massed enemy infantry. So effective was Adolphus’ field guns and his tactics for their use that soon enough every nation in Europe and beyond had adopted his innovations, and the art of war again changed dramatically.