Why did people stop using medieval castles for fighting and defence?
That rather depended on where you lived. The one thing that did NOT bring about the demise of the castle as a military defence was gunpowder. Few castles were ever breached by cannon and those that were were of the “lightweight” variety. There were many of these around at the end of the mediaeval period which had been built more as status symbols than as a proper defensive position. Bodiam Castle is a good example of this and it is clear from first sight that it was never intended to put up any kind of prolonged resistance despite its apparent trappings of the “typical” castle. Corfe Castle proved invulnerable to artillery as late as the English Civil War. The damage evident today is the result of “slighting” after capture and the degree of damage is directly related to the resistance offered. The fact that Corfe has been almost destroyed shows the spirit of its defence. Even then, it proved extremely difficult to demolish using carefully placed explosives. In England, the end of the W
Castles were a purely defensive weapon and couldn’t really be used against opponents. Their value went down as the availability of gunpowder went up. The methods of warfare also changed so soldiers became more mobile and less defensive. Being bottled up in a castle wasn’t a swift move because you could be starved out. Limited exits meant limited moves. That doesn’t mean that castles aren’t stil being used. They make good bases for operations. That is, until someone finds out where the base is and brings in heavy weaponry to take it out. Castles can’t stand against a modern artillery barage.
Castle’s became “outdated” mainly due to the advent of gunpowder. Cannon’s made it easy for advancing armies to destroy castle defenses. They were more powerful and had longer range. They were also much more mobile than large catapults. On top of that, the political structure of Europe (and other parts of the world) started to change over time. Feudalism was a very popular form of government across most of Europe in Medieval times. In the feudal system, a lord would control a small slice of the kingdom, and would need to defend it. Castles were best suited for that. But gradually nation states started to evolve, and as countries started to create national militaries, then the need for each feudal lord to defend his own slice of the kingdom became less, and the castles they used to defend their lands became less important.