A siege engine is defined as a device designed to break a castle’s fortifications or cause the circumvention of same. There were as many different types of siege engines as there were castle designs.
Largely constructed of wood, many used mechanical devices to launch attacks on the castle battlement by firing stones and other missiles that were intended to break down the defenses of the walls or to allow entry of the soldiers into the castle in order to facilitate the opening of the castle doors so that the attack could be pressed more force fully.
Until the siege engines were used, the only way for an army to get an army out of their castle was to lay siege around the castle until such time that the individuals inside either ran out of food and were forced to leave the castle and commit to battle or face the fear of starving inside the walls of the castle.
Another method used in the attempt to get the occupants out of the castle was to launch disease ridden carcasses of dead animals inside the walls in the hope that the disease would spread quickly among the people inside the castle walls, and this will force the remaining people to surrender.
Catapults and trebuchets were used to throw heavy material at the castle walls in order to affect a breach of the castle walls so that the attacking army could gain access to the army inside the walls.
Fires were also set against the castle walls in an attempt to compromise the integrity of the mortar that the stones of the wall were set into in the hopes of a breach. The occupants of the castle were not completely defenseless however.
They would pour burning oils and other materials on the attackers as well as employing archers along the walls to repel attackers and try to kill as many soldiers on the outside as well.