Why did the normans build motte and bailey castles

Motte and bailey castles appeared in England after the Norman Conquest of 1066. Motte and bailey castles were a common feature in England by the death of William the Conqueror in 1087. Their construction was the start of what was to become a massive castle building programme in England and Wales
When William landed at Pevensey in 1066, he immediately set about building a castle to protect himself and his most important men. William was a highly skilled and feared soldier who had learned his military skills in northern France.Some of the Vikings eventually stayed in northern France and the Norsemen became the Normans. They had been impressed with the French castellans and adopted them; the most popular design was the motte and bailey. In these castles, there was a fortified building (the castle) on top of a man-made hill called a motte. This served as a final fighting place where soldiers would retreat if the rest of the castle had been breached. The castle on the motte was reached either by wooden stairs that could be destroyed if the castle itself was attacked or by a ‘flying bridge’ that connected the bailey to the castle. In the bailey, people and animals lived in relative safety in times of peace as they were surrounded by a large wooden fence that kept out attackers and wild animals.