First gunpowder infantry unit of the game. Upgrades from the Longswordsman.
The Musketman is the first gunpowder unit in the game, and although it's not much more powerful than its predecessor, it is a necessary step to continue to the other, more powerful units in the late middle game. They are a step back from the massive over-protection of the Longswordsman, relying more on offensive power than steel defense. Their weapons are actually ranged weapons with a very short range - they need to enter the tile they want to attack, although they don't necessarily attempt to mingle with the opposing battalion like classical melees from the old times. Instead, they line up and fire their muskets at the enemy ranks, then, depending on the situation, reload to fire again (which tends to take quite some time), or use the attached bayonets to enter real melee combat.
The firing capabilities of the Musketmen allow them to damage more primitive units as they close to melee range; after that, they can use their muskets as short spears in melee. The total result gives them a slightly higher combat strength than the Longswordsmen, who have the edge in melee with their massive armor, but enter battle already worn down by the Musketmen's welcoming volley. Also, the Musketman doesn't require Iron like the Longswordsman or Horses like the Knight, allowing them to be built in larger numbers if strategic resources are scarce and making Pikemen and other anti-cavalry units less effective at repelling them.
The Musketman is available soon after the Longswordsman, and the requisite tech is in much the same part of the tech tree. For this reason, many offensive players choose to rush straight to the Musketman (researching Gunpowder immediately after Steel), as it is slightly stronger and provides an even greater advantage in battle.
The musket is a flintlock, muzzle-loading, long-barreled weapon which is fired from the shoulder. Muskets were lighter and more accurate than the earlier military firearm, the arquebus, and by the 18th century they had supplanted them on most European battlefields. With the addition of a bayonet the musket became a serviceable close-combat weapon as well, rendering the pikeman obsolete. The British army's "Brown Bess" was the archetypical musket, in service from 1722 until 1838; it was used by both sides in the American Revolution.
The proper term for a soldier wielding a musket is a "Musketeer." This unit was likely named "Musketman" to avoid confusion with the French unique unit with the same name.