Where did people build medieval castles?

Answer 1

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.

Answer 2

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.