Castles were put in different places for different reasons.
Some castles defended trade routes, and were placed to collect tolls. These were placed along roads, rivers, and important places on the shore. Dover Castle is an example. There are castles all along the Rhine of this type also.
Some castles were placed in the land holdings of important nobles for defense. They were sited according to what was best for defense among possible places owned by the lord. Loch Leven Castle might be regarded as one such.
Some castles were sited for defense and also to awe a local population. The great castles Edward I built in Wales are perhaps of this type. They were garrisoned for a few years, and they partly garrisoned. They only had military significance for a couple of decades because the people were no longer considered a threat. Harlech Castle is an example.
Generally on a high hill or motte. The people who built the castle wanted to be able to see a distance away so they could spot someone or an army approaching. Another area a castle was often high above a coastal area where it was impossible to climb up to the castle from the water, yet they could spot ships/armies approaching. Several were built on islands and they used the tides to provide access to the castle. Some were built against cliffs and were part of the cliff. Again, as a means of protection and to prevent access.
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Medieval castles.
People who lived in castles during the medieval times used them for attacking other castles and forcing them to surrender.
Medieval castles were needed for safety because they protected people from invaders and other dangers. Castles were also built to defend from danger. They were equipped with high walls, murder holes, cover for soldiers, and more.
To defend and protect people and important places of course.
Medieval times
they were expensive to build
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So that they could be protected during times of war
Cheap labor for defensive fortifications.
to protect there people
Medieval castles.
Peasants were not responsible for building castles in Medieval times. Masons and other trained professionals were in charge of making sure castles were built properly.
There is nothing different between todays castles and medieval time castles. Today's castles were built in medieval times.
They asked the Stonemasons to do it
The welsh medieval castles were big and gray.
Medieval castles were built by local lords who ruled the area. They owned the land, collected taxes and dispensed 'justice'. They required castles to protect themselves from unhappy locals, and sometimes from foreign invaders (depending on the country), and to display their power.