I've always been fascinated by the hundreds (thousands) of medieval castles all over Europe (as mentioned in another post). But puzzled why anyone would spend so much money to build these huge, expensive, fortified castles in a sprint of activity and then seemingly stop building them in the 14th and 15th centuries, and abandoning them shortly thereafter. Why were castles no longer useful past the early renaissance?
Why were castles no longer useful past the early renaissance?
Cannon. It's all very well having tall stone walls, but when your enemy brings a lot of cannon with him they're not going to do you much good, unless your walls are extremely thick as well. Engineers responded with the trace italienne, star shaped forts, with relatively low, thick, sloped walls, that could deflect cannonballs and spew flanking fire on almost every meter of wall.
As usual with military innovation, it made things more expensive. The new forts required artillery of their own and men with firearms to provide counter-battery fire and keep attackers from scaling the walls. This meant that fewer states/nobles could afford to build them.
source: Geoffrey Parker The military revolution
People built castles as a way of consolidating their hold on land. A castle was a place where you could base a small armed force and have it hold out against a much larger force until your main army could arrive. They also created towns and economic power around themselves. Castles were used in this way to particular effect in 11th-12th c. France and against the Welsh by Edward I.
Castles were no longer useful because of cannon. Cannon meant that walls could be overcome and a strongpoint taken much more quickly, reducing the effectiveness of the fortification.
For more see: R.W. Southern The Making of the Middle Ages p.85-88.