Belfries were built at a distance from the wall, then rolled into position. They were constructed with a series of platforms connected by ladders. Heavy planking and fresh animal hides helped protect the attackers from missiles and hot or burning liquid thrown from the castle. Once in position, the men in the belfry would lower a bridge and rush the defenses atop the wall. The ground level of the tower provided protection for sappers, or could house a battering ram.

It is unlikely the belfries in the illustration could close the distance to the castle; the approach is too steep. On level ground, defenders often prepared the ground around the castle to disable belfries. The defenders dug ditches and refilled them with soft earth that could not support the enormous weight of a siege tower. Attackers were forced to fill and compact the approach before advancing the belfries.