The Musketman is a considerable upgrade from earlier melee units: it's the first land unit to use firearms instead of close-combat weapons. The catch is that it requires a brand new strategic resource - Niter - and this one isn't very easy to find. However, you should really try, because the Musketman is great against all earlier units, even the Knight. Once you acquire it, your lines will strengthen sufficiently to assault Renaissance cities.
If an arrow or quarrel could stop an armored man, just think what a small, fast-moving ball of lead could do when it passed through the metal into the squishy bits below. Replacing the unwieldy arquebus as the preferred firearm in the early 1700s AD, the musket was a smoothbore, muzzle-loaded weapon triggered by locking mechanism that snapped shut to ignite a gunpowder charge – the archetype being the British “Brown Bess,” which served the Redcoats so well from 1722 through 1838. Sure, frontier settlers also used them around the world for hunting, protection and killing the natives, but it was in the wars of Europe that military tactics evolved to make use of the musket en masse in 'pike-and-shot' tactics such as the Spanish Tercio. In China the hand cannon had evolved into the musket, and the Ming and Qing dynasties made extensive use of them. But it was the Europeans and Ottomans who used them to best effect against each other, with elite units armed with muskets.