Cannon made Castles were obsolete for defense

Interestingly, the reverse is true of East Asia, where cannons favored the besieged and not the besieger. The reason for the discrepancy lies in wall construction:

In Europe, by contrast, new gunpowder weapons shifted warfare in favor of the besieger, a difference that requires some explanation. Medieval European castle walls were relatively high and thin (to prevent escalade), constructed of stone facings with rubble infill. Chinese walls were low and thick, built of hard-packed earth, with the later addition of brick facing. Even a very limited siege train of early cannon could quickly knock holes in a European castle’s walls. Moreover, those walls were not strong enough to support their own cannon, though they were able to permit the use of handguns. The defensive response to cannon in Europe was to build relatively low and thick walls of packed earth, which could both withstand the force of cannon balls and support their own, defensive cannon. Chinese wall-building practice was, by happenstance, extremely resistant to all forms of battering. This held true into the twentieth century, when even modern explosive shells had some difficulty in breaking through tramped earth walls. One of the reasons the Communist Chinese regime tore down Beijing's old city walls was specifically to prevent the old city from forming a defensive position that might be used to resist the government itself.

Source: The Asian Military Revolution by Peter Lorge (2008)

Cannon made Castles were 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.

For a brief while, warfare looked to become much more fluid and dynamic without the need to overcome the fixed defenses of castles every few miles.

By the 1520s, though, 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 castle had been made obsolete by cannon and a new type of fortification, more effective in defending against artillery, had been developed.

Star forts, however, were more expensive than castles and needed many more men (especially artillerymen) to man them. This accelerated a move to larger and larger armies, and the continued rise of national power (which could afford star forts and large armies) over the power of local nobles.