they were expensive to build
during late medieval times :p
in the late medieval period many medieval castles did have a primitive type of cannon.
Medieval castles.
There is nothing different between todays castles and medieval time castles. Today's castles were built in medieval times.
The welsh medieval castles were big and gray.
Medieval times
Castles were invented in medieval times.
There are no genuine medieval castles in the USA, because the Americas were not discovered until after the medieval period had ended. There are the "fairytale" Disney castles and some modern attempts at reconstructing medieval castles, but no genuine ones.
Who is most important in medieval history - kings and emperors. Where do they live - in castles.
Around the late 1300s, when siege cannon became strong enough to readily damage the castle.
Motte and bailey castles, Stone keep castles and Concentric castles
Medieval castles evolved because the old Motte and Bailey castles where easier to set alight and where not very strong, so they made stone castles to improve defence.