The traditional Medieval Castle was generally a fortified residence, offering protection for a knight and his family, and the inhabitants of the surrounding countryside. Larger castles were built by noblemen and kings. Most castles were residences, but the term was also used to describe fortifications built the same way (high walls, towers, a keep) for mostly military rather than residential purposes.

It became obvious that traditional Castles had become obsolete for defense during the "Italian Wars", 1494-1559.

These were a series of wars in Italy, involving the Italian city states and the French and Spanish.

When Charles VIII of France invaded Italy, in 1494, he brought with him the first highly mobile siege train of cannon. These soon proved to be able to very quickly defeat castles. The city of Monte San Giovanni, in the kingdom of Naples had its city walls and its castle shot to rubble after the defenders killed the French envoys. The French army then massacred the defenders. Many other, previously strong medieval castles and defenses were demonstrated to be equally vulnerable.

Castles had been shown to be unable to resist modern cannon.

By the 1520s, the "Trace Italienne" or "Star Fort" had been developed as an effective defense against cannon. Over the next 100 years, these forts spread across Europe and around the world.

The Star Fort had much lower, thicker, sloped walls, more resistant to cannon fire than the high walls of a castle. The star design and moats outside the walls were to protect the star fort from infantry assault (to which they were more vulnerable than high walled castles) by allowing cannon and musket fire from inside the walls to sweep the bases of all the walls and destroy attacking infantry.

Star forts, with their own cannon, requiring large cannon crews, were rarely built as residences. They were more often city, or purely military defenses.

The castle had been made obsolete by cannon and a new type of fortification, more effective in defending against artillery, had been developed.

http://militaryrevolution.s3.amazonaws.com/Primary%20sources/TraceItalienMR.pdf

So, what happened to the castles? Those that were residences, often still remained in use. Some fortification might still be a sensible precaution against local difficulties, even if it couldn’t stand up to an artillery train.

As times grew safer, many of these residential castles were modified or extended so that they became more and more like country mansions, and less and less like fortifications. Larger windows were added, non-fortified wings were extended, curtain walls were torn down and the stone re-used. Many such castles still exist in Europe today.

Other residential castles were abandoned. Either their defensive location (often high on a hill) was inconvenient, or the family decided to build a more ‘modern’ mansion somewhere on their estate rather than modify the old castle. These castles often still exist as ruins, but they were frequently quarried extensively for their stone once abandoned. Some disused castles were repurposed as farm buildings or barns.

Military castles, especially in strategic locations, were sometimes surrounded by star fort fortifications. Other military castles were abandoned, or converted to administration centers or other uses, where their inefficiency in modern defense was less relevant.

There were thousands of castles built across Europe, and many of them are still there to see today in all of the various states described above. Some are just the remnants of the mound of a motte and bailey castle. At others there might be foundations and a few crumbling walls or towers. Some are quite well preserved in pretty much their medieval state. While there are many that show later modifications ranging all the way from modest enlargement of windows and reduction of fortifications to mansions which look totally unfortified, but in fact are built around some of the walls or foundations of the original medieval stonework.