First created very far back, perhaps as early as the 7th century BCE in China, the crossbow was a variant projectile weapon that came to redefine warfare of the era. Bows, especially longbows, we considered a specialized weapon on the field. They requires not only dexterity in aiming the weapon but also great strength to pull the bow taut, giving the arrow enough pull behind t to send it flying with deadly, and accurate, force. Only certain trained soldiers could use bows, making a skilled bowman quite a prized addition to an army.
Crossbows, though, were different. Instead of the bow being held vertically, aligned with the body and requiring the strength of the user to fire, a cross bow was shorter, smaller, and used a gear-and-pully mechanism that could be wound and triggered. It's smaller bow, built crosswise along a frame, could be hefted by most, fired by even untrained conscripts, and easily used by all. It changed the game when it came to projectiles, meaning more fired shots could be aimed at enemies.
By design, crossbows were smaller and easier to use. They didn't have the range of a practiced longbowman, and their bows didn't even fire traditional arrows (instead shooting smaller arrows called bolts). But they enabled even unskilled soldiers the ability to strike the opposition from a distance, whatever that might have been, and their bolts could pierce armor, just like arrows, making them deadly and efficient.
Of course, the crossbow fell out of favor on the field of battle wit the arrival of firearms, rendering these efficient weapons (and the larger bows) moot.
Crossbows in Combat
The first, and so far only, appearance of crossbows came in Castlevania: Portrait of Ruin, with the crossbow being one of a number of sub-weapons hero Johnathan Morris could acquire on his adventure. Dropped, appropriately enough, by the Crossbow Armor, this sub-weapon could fire out a single bolt quite quickly. The bolt would fly, at high speed, across the screen, only near the end of its journey losing momentum as it slowly sank to the ground (not that most players would ever see it hit the ground as it would leave the edge of the screen long before then).
Quick on its attacks, the crossbow was a decent early- to mid-game weapon for a player that wanted to get it and master it. The bolt had a fire elemental placed on them, making them useful against enemies weak to that type. As a bonus, a mastered crossbow bolt would unleash a larger blast of fire when it struck dealing better damage. Despite these benefits, though, the crossbow was a little too slow, and a tad too weak, to really be useful into the later parts of the game.