I thought you might be interested in learning more about the Normans and their castles that the Welsh Prince Cochgam ap Cadwgon has to contend with in The Archer's Diary - Book Two The Journal.
It wasn't long after their conquest of the English at the Battle of Hastings in 1066 that the Normans quickly spread throughout the country establishing their control over the land and its people by building castles in as many strategic locations as possible. All in all, it's believed they built over 1,000 castles during the Middle Ages (medieval times.)
These first castles were not the stone structures you are most likely familiar with depicted in Robin Hood movies, but were constructed mainly of wood and earth and called motte and bailey castles.
The motte and bailey castle was an early form of medieval fortification especially popular with the Normans in northern France and Britain during the 11th century CE. A single tower was built on (or partially within) the motte or earth mound while a courtyard area or bailey at the base was protected by a wooden palisade and an encircling outer ditch. Relatively quick to build, the height of the mound made the tower difficult to attack while the wall offered a place of refuge from opportunist raiders. For these reasons, the motte and bailey castle was especially useful in freshly conquered territories where the native population was still hostile to their new overlords. As stone resisted fire better than wood and defensive designs improved, castles evolved into more permanent structures with stone circuit walls and towers enclosing a more impressive inner stone tower or keep (donjon).
Evolution & Design
The earliest form of fortified camp was a simple wooden palisade, perhaps with earthworks, surrounding a camp (ringworks), sometimes with a permanent wooden tower in the centre. These had been common since Roman times and remained little-changed for centuries. Then, stand-alone wooden towers became a feature of defences in northwest France from the 9th and 10th centuries CE. These structures evolved into the more sophisticated motte and bailey castles, which were especially common in France and Norman Britain from the 11th century CE.
The castles consisted of a wooden wall, perhaps built on an earth bank, encircling an open space or courtyard (bailey) and a natural or artificial hill (motte) which had a wooden tower built in the centre of its flattened top, sometimes surrounded by its own wooden palisade. The tower ranged from a mere lookout tower or firing platform to the more substantial building used as a residence for the local lord. Some towers were built on stilts, presumably to save time and materials in their construction and to make them more difficult to scale. The motte was sometimes connected to the bailey by a type of bridge, but most had steps cut into their sides.
The whole castle structure was further protected by an encircling ditch, which could be with or without water. There was no specific design blueprint to follow as castles took advantage of local terrain and other factors, as the historian N. J. G. Pounds here notes:
With variations in dimensions, layout, towers, walls, and foundations, some castles had two mottes while some mottes had two or even three baileys. There is also sometimes the difficulty, given the lack of surviving structural remains, of distinguishing a fortified private home built on a mound from a castle used as an administrative centre. The latter were generally larger and the bailey therein typically contained domestic buildings, stores and supplies, workshops, stables and, crucially, a well.
Cartwright, Mark. "Motte and Bailey Castle." Ancient History Encyclopedia. Ancient History Encyclopedia, 17 May 2018. Web. 25 Jul 2020.