An unusually large siege tower, as used at Courcy in the 1090s. By the 14c, they had become movable on wheels. Many were tall enough to allow bowmen to fire on the place besieged at eye-level. They were also referred to in OldFr. as a *berfrois, being used as a watch tower. The tower was perhaps named after a belfry, which in the Latin of the time was called a berefredus. 'Belfry' was used equally of a military tower and the bell tower of a church, attached or separate. The word's second element, -fredum or -fredus, may well be derived from *frith and carried a sense of protection.
Cf. Sow
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.