There were many ways of attacking a medieval castle, and almost as many means of countering such attacks.
Ways to Attack
The first step was to batter the walls with a variety of “engines”, the most common being the catapult or mangonel. Its throwing beam would haul a massive stone, a pot of flaming “Greek fire”, a dead horse (which might infect the garrison) or sometimes… a captured messenger (to show that all hope of relief was in vain).
Another popular method used, was undermining. Beginning some distance away, miners burrowed beneath the defences, supporting their tunnel with wooden props. They then filled the mine with combustible materials – such as, the fat of half a dozen pigs – and fired them, burning away the supports, collapsing the tunnel and, with luck, demolishing the wall above it.
If the castle was founded on solid rock or was surrounded by a moat, undermining was a useless method and in these cases it was often necessary to storm the walls instead by using ladders or a “belfry”. A belfry was a wheeled tower with the uppermost platform being the same height as the top of the castle walls. Another method would have been to assault the wooden gate of the castle with an iron-headed battering ram, swung on a sturdy frame.
A traitor within the walls of the castle was the second most effective weapon, whereas, the most effective was cutting off the supplies to the castle. All castles had a well, but if this dried up or was poisoned, the defenders ultimately had to surrender.
Countering the Attack
If the garrison suspected that the castle was being undermined, they could sometimes locate the underground workings by standing jugs filled with water in different parts of the fortress and observing them when they vibrated. They could then sink a counter-mine from inside the castle, and either slaughter the opposing diggers with hand-to-hand fighting, or fill their tunnel with water.
The people defending the castle (this was usually any able person, not just the garrison) knew that eventually the walls of the castle would be attacked. In defence they would use anything that came to hand to stop the attackers breaching the walls. “Firepots” were dropped from above onto the people scaling the ladders or using the battering ram, along with javelins, stones, boiling oil and a scalding oatmeal mush which stuck to besiegers’ skins.
If the attackers managed to get within the outer gate, they probably found themselves trapped in a passage between two portcullises, here they would be showered with missiles from “murder holes’ in its roof.