Defending an expensive medieval castle required excellent engineering skills. Castles were the most expensive buildings in Medieval England and no owner - be they a king or a noble - could afford to lose one. Therefore, great ingenuity was put into castle defences.
As castles were static, an enemy could usually get close to the actual buildings. Therefore outer walls - curtain walls - were built as a first line of defence. If these were broken into, then the castle itself had many defensive features.
Castle builders realised that round towers had more strength than conventional square ones. A traditional way of defeating a castle had been to dig a tunnel underneath one of the towers in a castle's keep, fill the end of the tunnel with anything that could burn and cause the tunnel, and then the castle's corner, to collapse.
A round tower at
Pevensey Castle, East Sussex
Another obvious way to stop an advance on your castle, was to build a moat around it. Though these may appear a robust form of defence, they could also give an enemy a very easy passage to the castle's walls as all they had to do was to cross the moat at its weakest point - though this would be done with the defenders attacking the invaders!
The remnants of a water-filled ditch at Pevensey
In the castle itself were usually a variety of defensive mechanisms. To stop the enemy actually getting in, the entrance to the castle was heavily fortified. Known as a barbican, this part of the castle would have a drawbridge, a portcullis, arrow slits, machicolations (murder holes) - any devise that was thought to be useful at stopping the enemy.
The gap where a portcullis would drop to the ground
Many castles were very well defended and for some attacking armies, the only way to defeat them was to surround them and starve them out. This was potentially a very long process with no guarantee of success. It was the development of mortars and cannons that brought an end to castle building as they were no longer financially viable to build as they could be easily destroyed.
"Defending a castle". HistoryLearningSite.co.uk. 2014. Web.