A belfry or siege tower was used to attack the castle walls during the final days or hours of a siege or battle. Once the garrison was "softened up" by the bombardments of the siege artillery, the belfries would roll up to the castle wall and open its top gate. The attackers would climb up the siege tower while being protected and storm the walls. If there was a moat, the attackers would fill it in and create a land bridge to the wall. At the same time, another group of attackers would storm the gatehouse in an atttempt to take the castle that way. to Hopefully, the charges would be a success. Sometimes, it wasn't. In the later years of the Middle Ages, people invented ways of rigging a bucket full of burning tar on the end of a long pole so they could safely attack the belfry while on the other sides of the wall. There were also hooks that could latch onto the bottom and upturn the thing. So even though the belfries were a very valuable weapon, they could be destroyed by the simplest method.
