A crossbow is a projectile weapon which is commonly used by warriors because of it's power and accuracy.
History
During the Battle of Ealdor, brigands used crossbows while they were on horses (The Moment of Truth).
When Arthur,Merlin, and a set of knights went hunting, Arthur used a crossbow to kill a Unicorn. This brought a terrible curse upon Camelot (The Labyrinth of Gedref).
The assassin Myror tried to kill Arthur with a crossbow but was unable to do so since the prince was not present (The Once and Future Queen).
While trying to fight off the dragon, the Knights of Camelot used crossbows, which proved to be ineffective as the dragon was able to dodge the bolts (The Last Dragonlord).
Camelot used crossbows again during the Camelot-Essetir war (The Tears of Uther Pendragon).
Julius Borden used a crossbow to attack the Knights of the Round Table and succeeded in hitting Percival in the leg before Merlin broke his crossbow (Aithusa).
Merlin, having been turned into an assassin by Morgana, asked Sir Leon for a crossbow so he could use it to kill Arthur. He found one, but Leon told him that it was ancient and probably wouldn't hurt a fly. He then gave Merlin what he called a "thing of beauty", carved from 50-year old ash. Merlin had an impressive shot when he destroyed a barrel with the crossbow. When Merlin told Leon that he was going to use it to kill Arthur, he assumed he was just kidding (A Servant of Two Masters).
The crossbow was also used when Arthur, Mithian and the knights went hunting (The Hunter's Heart).
During Morgana's second reign, Arthur and Merlin were posing as smugglers. Agravaine eventually tracked them down, so they used crossbows to defend themselves (The Sword in the Stone).
Trivia
- Other then magic, Merlin appears to be able to use the crossbow with ease.