There are a great number of archaeological and natural geographical features which appear as mounds of earth. The difference between even a well preserved large Bronze Age bell barrow and a small isolated Medieval motte is difficult to tell and when sites are eroded and otherwise damaged this difficulty can only increase. The personal biases of the sites describer, however unconscious, may well be the most important factor in a given assignment for a monument. Even sites which have been excavated may actually only have had a small test pit dug and the excavation may well not have established the complete history of a site. Sites can have a complex history of use in many periods and a site with several hundreds of years use as a focal point for a Bronze Age community, a long period of neglect, a dozen years use as a small Norman motte and a few hundred years use as a millstead may not show any signs of Norman military or residential occupation, particularly since few pre-modern people were as free with their rubbish as current western culture. In such circumstances a cautious judgment has to be made and this judgment needs to be informed by as wide a set of example as possible, which is one of the reasons for this web site.
Probably the best defining feature of a motte is a surrounding deep ditch. Originally, according to medieval documents, these ditches were supposedly deeper than a single man could throw the excavated soil, that is greater than 2 metres. In practice not all castles had deep ditches, since marshy land or particularly stony or rocky land would have made the digging of such ditches either impossible or too time consuming. Even where ditches were built these can rapidly fill in. A ditch dug in a hard sub-soil or a rock cut ditch may survive many centuries only slowly filling in with blown in soil but a ditch in soft soil, particularly if in a flood plain of a river, can disappear in just a few years as soil is washed in by heavy rain or flooded in from the river. Here all that may hint at the previous existence of a ditch might be an area of slightly more lush grass with more mole hills than usual around a much eroded mound. Deep ditches may have preserved a motte from being damaged by horse drawn ploughs but modern tractors, with deep ploughs, have destroyed a number of mottes and their ditches and the mound recorded on an old map will probably not give enough detail to be certain of an identification.
Mounds are also subject to damage from other activities than agriculture. Since they tend to be above the water table they are favoured by animals which dig burrows such as rabbits and badgers. It is easier to dig soil that has already been dug rather than fresh soil so mounds can be quarried for soil or gravel for road repairs etc. Gravity means than erosion from animals walking on the mound or just from heavy rain will result in soil moving downhill and the mound becoming flattened. It may well be that some (and possibly many) mottes originally started with very steep or even vertical sides, revetted with timber, hazel hurdling or dry stone walling but erosive forces on such features are particularly strong and soon turns such mounds into a more conical form when the revetment fails.1
A particularly difficulty with identifying castle mounds arises with C17-C19 prospect mounds. These were mounds designed to aid the viewing of the extensive and expensive landscape gardening of the great country houses. However, many of these great houses were successors to timber castles and many prospect mounds were actually the nearby mottes of the original castle. Thus, in a style and fashion conscious world, the form of prospect mounds was very similar to that of (eroded) mottes and great houses without a nearby motte would build a prospect mound of such a form. Thus genuine mottes reused as prospect mounds and new prospect mounds can have very similar positions and forms and be virtually indistinguishable. Pictured here is a mound at Shackerstone, Leicestershire, this has been identified as a motte but Oliver Creighton believes it to be a prospect mound. It can certainly be seen that this has been adapted as a prospect mound, with a nice spiral ramp but the ditch (on the other side of the mound from this photo) is deeper than a prospect mound warrants and the position very close to the church, within a possible bailey site obscured by modern building, means that this mound may well have originated as a motte.
A similar issue arises with some barrows. Some prehistoric burial mounds maintained a position as a focus for communities during and after the Roman conquest. Early Christian missionaries in the C6 and C7 deliberately used such 'pagan' features as a focus for their new churches. Thus we can have a situation where a prehistoric mound was a focus for an early Saxon community and then a location for a new church. The local Saxon leader, having been baptised, might well move his hall close to this new church giving it the modest fortifications of the Saxon thegnal burh. After the Norman Conquest a new Norman lord may well have reinforced this thegnal burh with deeper ditches and by incorporating the barrow into the defences as a motte.
A further issue lies with collapsed buildings. A collapsed building fairly quickly gets covered with plants and starts to get soil involved in the rubble, these plants die and compost down to more soil, after a few centuries the collapsed building looks very much like a mound of earth. Even if some bit of masonry remains sticking out of the rubble it might well look like a building on a mound rather than a building surrounded by rubble. A fine example is Pevensey Castle where the collapsed keep was described as a motte by Clark before it was cleared in the early 20th century.