<P> Various methods were used to build mottes . Where a natural hill could be used, scarping could produce a motte without the need to create an artificial mound, but more commonly much of the motte would have to be constructed by hand . Four methods existed for building a mound and a tower: the mound could either be built first, and a tower placed on top of it; the tower could alternatively be built on the original ground surface and then buried within the mound; the tower could potentially be built on the original ground surface and then partially buried within the mound, the buried part forming a cellar beneath; or the tower could be built first, and the mound added later . </P> <P> Regardless of the sequencing, artificial mottes had to be built by piling up earth; this work was undertaken by hand, using wooden shovels and hand - barrows, possibly with picks as well in the later periods . Larger mottes took disproportionately more effort to build than their smaller equivalents, because of the volumes of earth involved . The largest mottes in England, such as Thetford, are estimated to have required up to 24,000 man - days of work; smaller ones required perhaps as little as 1,000 . Contemporary accounts talk of some mottes being built in a matter of days, although these low figures have led to suggestions by historians that either these figures were an underestimate, or that they refer to the construction of a smaller design than that later seen on the sites concerned . Taking into account estimates of the likely available manpower during the period, historians estimate that the larger mottes might have taken between four and nine months to build . This contrasted favourably with stone keeps of the period, which typically took up to ten years to build . Very little skilled labour was required to build motte and bailey castles, which made them very attractive propositions if forced peasant labour was available, as was the case after the Norman invasion of England . Where the local workforce had to be paid--such as at Clones in Ireland, built in 1211 using imported labourers--the costs would rise quickly, in this case reaching £ 20 . </P> <P> The type of soil would make a difference to the design of the motte, as clay soils could support a steeper motte, whilst sandier soils meant that a motte would need a more gentle incline . Where available, layers of different sorts of earth, such as clay, gravel and chalk, would be used alternatively to build in strength to the design . Layers of turf could also be added to stabilise the motte as it was built up, or a core of stones placed as the heart of the structure to provide strength . Similar issues applied to the defensive ditches, where designers found that the wider the ditch was dug, the deeper and steeper the sides of the scarp could be, making it more defensive . Although militarily a motte was, as Norman Pounds describes it, "almost indestructible", they required frequent maintenance . Soil wash was a problem, particularly with steeper mounds, and mottes could be clad with wood or stone slabs to protect them . Over time, some mottes suffered from subsidence or damage from flooding, requiring repairs and stabilisation work . </P> <P> Although motte - and - bailey castles are the best known castle design, they were not always the most numerous in any given area . A popular alternative was the ringwork castle, involving a palisade being built on top of a raised earth rampart, protected by a ditch . The choice of motte and bailey or ringwork was partially driven by terrain, as mottes were typically built on low ground, and on deeper clay and alluvial soils . Another factor may have been speed, as ringworks were faster to build than mottes . Some ringwork castles were later converted into motte - and - bailey designs, by filling in the centre of the ringwork to produce a flat - topped motte . The reasons for why this decision was taken are unclear; motte - and - bailey castles may have been felt to be more prestigious, or easier to defend; another theory is that like the terpen in Netherlands, or Vorburg and Hauptburg in Lower Rhineland, raising the height of the castle was done to create a drier site . </P>

Where did the normans build castles and what did their castles look like