These castles are called "motte and bailey" after the peculiarity of their design. The major strongpoint, the keep, was built on the top of a flat-topped conical mound, the "motte", surrounded by a ditch and embankment, with an area of ground attached, also protected by at least one ditch and bank, the "bailey". Whilst the ditches and banks were man-made, the motte could either be man-made, or a suitably sited natural feature. The banks would have been topped with palisades or walls, with the usual complement of towers and gates. The bailey provided a secure site for the ancillary domestic buildings such as hall, chapel, stables etc.

It may be incorrect to say that this style of castle design was only introduced into England with the Norman Conquest of 1066. England was not so isolated either politically or culturally from the mainland of Europe that such things were completely unheard of. My references point to a few sites which are clearly stated to be 'pre-Conquest'. What is most certainly true is that the Conquest brought about a massive increase in the number of such castles.

Most of these were built as a temporary means of subduing a nation which would not take kindly to Duke William of Normandy forcefully making good his claim to the English throne. To this end, William shipped large numbers of castles from Normandy to England as wooden prefabricated kits of parts. Once the earthworks had been constructed, these temporary wooden keeps were erected on their mounds and served a similar purpose to the turf-built forts of the earlier Roman occupation.

In due course these castles were no longer needed and were dismantled, leaving only the earthworks behind. Many towns and villages which existed at the time still display the physical remains of these structures. There were, however, a number of sites of particular strategic importance, and it is certain that the castles built there were designed to be permanent fixtures. That is, they were built in masonry from the beginning.

Whilst there were no great engineering problems concerned with building stone walls and towers on the tops of the embankments, the same was not necessarily true when it came to building the stone keep on the motte. If the motte was a natural feature, then the ground may have been compact enough to take the force of many tons of stone resting on it. If the motte was man-made, then any stone keep built on top of it would have collapsed in short order.

To overcome this problem, the castle builders adopted two solutions. The first was to minimise the amount of masonry by building a "shell-keep". This was basically a simple circular wall enclosing the top of the motte. The accommodation was formed by building lighter structures against the inner face of this wall, leaving an open courtyard in the middle. This alone would not have completely avoided the problem of subsidence, and there is archaeological evidence to show that the foundations of the shell-keep could be at the natural ground level and the wall and mound raised together, effectively giving a very tall masonry structure with most of its bulk buried within the mound.

Although my illustration shows the classic "motte and one bailey" design, this was by no means the only form. Many castles had two or even three baileys. In the case of a castle with two baileys, the military thinking of the time often resulted in the "hour-glass" design, the two baileys being arranged either side of the motte.

Two examples of this are Arundel Castle in Sussex, and Windsor Castle in Berkshire. Whilst both castles are today what I would classify as "Stately Homes", having been quite extensively rebuilt and modernised, their distinctive plans show quite clearly their ancient origins.

There are two important variations on the motte and bailey style. The first we can call the "Motte Castle" or just plain "Motte", because that is what it is: a motte without a bailey. The second variation is a "Ringwork", or a bailey without a motte.

Neither of these forms is as complete a defensive structure as the motte and bailey castle itself, so why build them? I would venture to suggest that several factors would have been at work here.

The ringwork, a ditch and palisaded embankment surrounding a homestead or farm, harks back to much earlier times and is an ancestor of all other castle forms. It is also comparatively cheap and quick to build. In areas where the threat of attack was fairly low then this would have been quite adequate. Neither would it have needed royal consent for its construction. In addition to being a defensive structure, it would also indicate the status of the site and its occupant to any passers-by.

The solitary motte, with a wooden or stone tower on its summit, is a slightly different beast. Perhaps we could think of it as being similar to the "vicar's pele", or watchtower, found in large numbers in the north of England. Although needing greater resources for its construction, it would still be cheaper than a complete castle. It could act as a watchtower in times of trouble and be a place of refuge in the event of an attack. Although any other buildings around it would be unprotected, the tower on the motte would be stronger and easier to defend than a palisaded enclosure. As regards status, the motte would be a step up. It would also give a certain message to any potentially unruly citizens, particularly those of a recently conquered nation, a message that could be seen for some distance.

It might be tempting to suggest that these two structures would be earlier in date than motte and bailey castles, but this would be a mistake. It is most likely that all three variations would have been constructed and occupied in parallel to one another. After all, there are many motte and bailey castles which were constructed at a time when they had been superseded by more elaborate stone-built edifices. If you were a minor lordling with poor quality land and a small income, then you built what you could afford.