instruction: Give a single-sentence explanation of this paragraph without quoting from the text.
context: 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.
response: Soil type and composition play a significant role in the kind of mound that can be built to support a motte and the maintenance required to upkeep it.