A moveable tower of wood on wheels, used to assault fortress walls, by placing a bridge from the belfry to the battlements so the attackers can arrive within combat range without having to resort to scaling ladders. It is named for its resemblance to the belfry of a church.
Upper storey of a tower or area in a steeple, in which one or more bells are hung. The photo on top is of the belfry tower in Ghent, Flanders. Regarding castles, a belfry is a mobile siege tower made of wood. It was moved up to the castle wall (with difficulty!) and a drawbridge like platform dropped onto the battlements to allow the attackers access.