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Regulars are not available to the player.
Europe's early firearms - the arquebus and the musket - were notoriously inaccurate. Even with the development of professional armies in the 17th century, when soldiers were allowed to train on a regular basis with their own weaponry, actually hitting a specific target with such a firearm remained a big challenge: units relied on mass volume of fire rather than on marksmanship to do their damage. For this reason, among 17th century armed forces it was not a regiment's ability to fire accurately that gave them their reputation - it was their ability to be fired upon and continue fighting. Regulars are just such units - hardened, disciplined, and deadly.