For over two centuries, the Motte and Bailey Castles were extremely popular. The use of wood as the principal building material was one of the many factors that contributed to the design's popularity (which was cheap and easy to use). Furthermore, lumber rots readily, and many of these early castles quickly fell into ruin. Finally, wooden buildings are less expensive than their stone counterparts.
The first true castles appeared around 1100. At this time, military leaders began to realize that towers and walls weren't enough to protect their settlements from attack. The only way to stop invaders from burning or pillaging your town is with a castle! However, since money was still used rather than coins, these castles were not luxurious (in fact, they often lacked any kind of interior decoration). They were large structures, built by invading armies from France and England to protect themselves from local tribes.
Over the next few hundred years, more sophisticated ideas about castle architecture began to emerge. Leaders realized that they needed something stronger and more defensible than a pile of stones covered in earth. They also wanted to be able to see approaching enemies at a distance, so they added towers on top of towers. Some even included drawbridges and gates for defense.
Around 1350, soldiers of fortune started to appear in Europe. These men went on raids parties seeking adventure and profit. In order to attract them to your town, you had to build them something spectacular.
Castle designs have evolved over time. This is due to technological advancements as well as changes in the function and purpose of castles across time. The early castles were merely earthen mounds, and medieval castle designs expanded on these fundamentals by including ditches in the Motte and Bailey style. As technology advanced, so did the design of castles. Modern skyscrapers use advanced engineering techniques that allow for tall buildings with small footprints. Castles can be built using similar technologies, so they too have been improved upon over time.
Living conditions in castles were also a factor in how they were designed. In the early days of castle building, military might was all that mattered- anyone who could afford a castle wanted to show that they had this ability. Thus, the first castles were large and unwieldy- it took a lot of manpower to build them and maintain them. As kingdoms became more established, they needed smaller castles for their police forces and other civil servants. These new types of castles were often located in isolated places where people wouldn't find shelter elsewhere- high on a hill or under a tree. Trees were even used as material for castle walls until modern times when steel and concrete took their place.
As time went on, new technologies were invented that again changed how castles were designed. Firearms changed the way wars were fought, so defenders of castles began to install guns into their fortifications.
Motte-and-bailey Initially, most of William's castles were crude timber motte-and-bailey structures, but they were quickly rebuilt to exceedingly spectacular stone keep castles with the most recent Romanesque architecture. The earliest surviving stone keep is at Alnwick in Northumberland; it was built around 1180 by Henry II for his son William. The technique of building with cut and polished uncarved rock instead of wood allowed the architects to create intricate designs in the stones that could not have been done otherwise.
William I also built several castles used as temporary shelters during warfare or raids: a tent made of fabric can be set up easily in just a few hours, while a wooden structure would take much longer to build. These include a large wooden fort near the future site of York that he may have used as a raid base before attacking Ireland in 1098, and another near Winchester where he may have stayed after defeating the king of England in 1066.
Motte-and-bailey and tent raids are only two examples of how wartime castles served important functions that required new buildings to be constructed quickly. Others included protecting valuable prisoners or hostages, preventing runaway slaves from reaching home, providing shelter for people displaced by war, etc.