A cascabel is a subassembly of a muzzle loading cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

 - a place to attach arresting ropes to deal with the recoil of firing the cannon.

Generally comprising the knob (A) and the neck (B), with particular models also featuring a filet (C). By some definitions, the cascabel additionally includes the base of the breech (D). Cascabels varied in design and appearance, and were a common feature of cannons from the 17th century until the advent of the breech loading cannon in the late 19th century.