Most castles, by the time of concentric castles, had what were called "murder holes" above their main entrance. Strictly called machicolations, defending soldiers above the murder holes would throw down onto the enemy underneath boiling water, boiling pitch etc.,  whatever was likely to put off the attacking soldiers. The attackers had to get close to the castle to get in, so they had to get under the murder holes to attack the main entrance. We do know that parts of dead bodies were thrown through these gaps to put off the enemy !!

Stone castles could have very thick walls as this photo shows. Battering rams etc. would have had a difficult time against such thick walls. The picture also shows battlements. These are strictly known as crenellations and gave defenders something solid to hide behind when they were not firing out from the gaps in between the stone battlements. In time of war, these gaps would have wooden shutters in between to give the defenders even more protection. Sometimes, very few soldiers were needed to defend a well built castle.

Again, the thickness of castle walls can be seen even if this castle, like so many others, is falling into ruin. This also shows the wall walk (parapet) along which defending soldiers would patrol. What is left of the crenellations can also be made out. 

To aid defence, the keep was always built above any castle wall (called curtain walls) so that the heart of a castle could see what was going on around it. Curtain walls were built progressively lower the further away they were from the keep. This allowed the defenders to   fire on attackers as they could always see them. Some castles were so well built, that attackers could only get to a castle through what was charmingly called the "killing field" - because the attackers had no-where to hide.

Round towers on a keep or on a curtain wall assisted defence as it was believed that any form of missile thrown or catapulted at the castle would deflect off of a rounded surface. Also if the enemy tried to dig under a castle to undermine a weak corner, they would not find a corner on a round tower ! So this form of attack was useless against a castle with rounded towers such as Beaumaris in Anglesey in Wales. Rounded towers could also contain a spiral staircase as is the case at the Tower of London

A defender's view of machicolations. Below these gaps the attackers would have to face boiling liquids etc. Defenders simply poured anything between the gaps.