A slight univallate hillfort is an enclosure situated on the top or towards the top of a hill, defined by a single line of earthworks, which entirely surround an area measuring between 1 and 10ha. The scale of the earthworks, which can comprise a ditch, a rampart and a counterscarp bank, is relatively small.
The hills where monuments of this class are located will not necessarily be the highest or most inaccessible in the region. For example, some slight univallate hillforts occur in low-lying areas, such as the Fenlands, on pieces of ground which rise only a few metres above sea level and the surrounding countryside .
The shape of hillforts belonging to this class varies considerably and includes examples of circular; sub-circular; oval; pear-shaped; sub-rectangular; triangular; trapezoidal; polygonal; and irregular ground plan.
The earthworks are usually broken by one or two entrances which are either marked by simple gaps, or by an inturned rampart or ramparts. Postholes revealed by excavation indicate single and dual portal gateways. More elaborate features, including overlapping ramparts and outworks, are limited to only a small number of examples.
Structures within the hillforts include square to rectangular buildings supported by four to six postholes; round-houses, built either of timber or stone; large storage pits, which are usually limited to sites in southern England; hearths; and a variety of scattered postholes, stakeholes and gullies.
Some confusion might arise from the apparent overlap between the smaller hillforts and defended settlements. In an attempt to anticipate this problem, size is used as a distinguishing factor. The lower limit of 1ha has been chosen because it reflects a break-point between a cluster of defended enclosures which surround an area of less than 0.7ha and a second group which exceed 1ha. As with most classificatory systems the clarity of this distinction is blurred by a few sites which fall between the two groups.
It might also be possible to confuse slight univallate hillforts with causewayed enclosures; Neolithic enclosures; and henges. In each of these cases the earthworks tend to be circular, oval or elliptical. Confusion is only likely to arise if the hillfort is of a similar shape, and if one of the other monument classes is located on a hilltop or upper slope of a hill. Distinctions between causewayed enclosures and slight univallate hillforts may be difficult if the hillfort rampart has been dispersed, and if the segmentary construction of a causewayed enclosure is obscured. Even if the causeways of the latter class of monument are no longer visible, the outline of the ditch should be far more irregular than would be the case with a slight univallate hillfort. Neolithic enclosures with continuous ditches are rare. Those marked by open-ended C-shaped boundaries can be easily distinguished from slight univallate hillforts, where the ramparts will describe a complete circuit. On Neolithic enclosures which are entirely surrounded by earthworks the ditches tend to be narrower and the banks smaller than the boundaries surrounding a slight univallate hillfort. Henges can always be distinguished by the location of the bank on the outside of the ditch; an arrangement which does not occur on slight univallate hillforts.
Slight univallate hillforts have variously been interpreted as stock enclosures; redistribution centres; places of refuge used only in times of crisis; or as settlements occupied on a permanent basis.