During the Middle Ages, castles were very important military features.  This meant that any conquering force had to be able to take a castle in order to dominate the area of the castle.  Castles were generally taken through siege warfare.  There were a number of tactics used in siege warfare.  Among them were:

• Using siege engines.  Attackers could use catapults to hurl heavy stones at the walls of the castle.  They could also use things like siege towers to get attackers up to the tops of the walls without being as exposed to enemy fire.
• Mining.  Attackers could try to dig under the foundations of the castle wall.  They could then collapse the tunnels they had dug.  This would cause the wall to weaken and fall.