Crossbows were a type of weapon commonly used in the Middle Ages. They were a type of projectile weapon. Specially-designed bolts were the usual ammunition.
In 1215, at the behest of the Master, a guard at Fitzwilliam Castle killed Sir Geoffrey de Lacy by firing a crossbow at him. He was hit in the back while riding from the castle and died from his wounds some time later. (TV: The King's Demons)
The Sevateem used the crossbow as their primary ranged weapon. Leela was proficient in its use. The Fourth Doctor used a crossbow to shoot a pulley, preventing his descent into a pit of Horda. He credited William Tell with his knowledge of the weapon. (TV: The Face of Evil)