This building allows basic military units to be recruited.
It is extremely useful for all governments to to be able to keep a visible military presence in the country, a reminder that "the rod and the axe" are available, should they be needed. This is true even in countries where liberty and freedom are trumpeted loudly: too much liberty, especially among the lower orders of society, is a dangerous thing. It is equally important to keep soldiers apart from the common folk: disorder and crime nearly always follow when they are allowed to mix.
Historically, most military cantonments or encampments had a distinctly non-military appearance. Most armies had a huge train of sutlers to provide food and drink, wagon masters for transport, armourers and smiths for weapons and gear, pioneers, holy men, hangers-on, hawkers and officers' servants. Alongside these was another army of wives, children, and the inevitable prostitutes plying their trade to officers and men alike. States recognised that these people were necessary to the smooth running of armies, but that did not mean that they made life easy for them, or looked after them in any way. On campaign, units had an allowance of wives, but those women left behind would have to shift for themselves and hope for a portion of their men's pay.
Requires 750 gpld, 3 turns, and an empty region capital building slot. Can be upgraded to Barracks.
Cantonments are the most basic type of army building. Cheaply and quickly built, they allow for the training and speed the replenishment rate of the most basic army units. The construction of cantonments is also required to access the most basic military technologies, thus opening up the tech tree for more advanced innovations further down the tech tree. Nearly all factions in all campaigns begin their campaigns with at least one cantonment (or a higher level army building), though, so the tech tree is almost never restricted for lack of a cantonment.