Unlike cavalry and artillery, infantry tends to have balanced effectiveness in both shock and fire, although the fire modifier is 0 until Land Tech 17 and isn't really meaningful until it reaches. Infantry are essential to any siege if you wish to make an assault. These units will perform better when defending in rugged terrain and worse when attacking in open terrain. Along with being the quickest unit to recruit, they are also the cheapest to purchase and maintain. Infantry regiments are the basic unit of your military.
