Why did the castle era change?

Answer 1

Castles were expensive to build, they were expensive to equip, their garrisons were expensive, and they fell apart of their own accord if they were not maintained at some expense. Once cannons were made useful and widespread, castles had lost a lot of their appeal.

Answer 2

Early castles were made of wood. People then started to construct castles out of stone because invaders could light wooden castles on fire, and stone was a lot stronger than wood.