From the National Heritage List for England entry:

The moated site at Groombridge Place comprises a stone-lined nearly square and broad moat defining an island 40m by 50m. The southern side of the island is occupied by a fine Jacobean house and associated service courtyard (both listed Grade I), but the north side is clear of buildings. The scheduling does not apply to any of the buildings on the site, although the ground beneath each is included. Moated sites are generally seen as the prestigious residences of the Lords of the Manor. The moat not only marked the high status of the site but also served to deter casual raiders and wild animals. Most moats were built between 1250 and 1350, and it is to this period that the moat at Groombridge is likely to date, since it first enters the historical record as a manor in 1286. No traces of the earlier buildings on the site are to be seen, but the sandstone lining of the moat probably represents early building material. The brick-built gatehouse and bridge on the north side, both excluded from the scheduling, may preserve the position of the original access to the moat.

Reasons for Designation
Around 6,000 moated sites are known in England. They consist of wide ditches, often or seasonally water-filled, partly or completely enclosing one or more islands of dry ground on which stood domestic or religious buildings. In some cases the islands were used for horticulture. The majority of moated sites served as prestigious aristocratic and seigneurial residences with the provision of a moat intended as a status symbol rather than a practical military defence. The peak period during which moated sites were built was between about 1250 and 1350 and by far the greatest concentration lies in central and eastern parts of England. However, moated sites were built throughout the medieval period, are widely scattered throughout England and exhibit a high level of diversity in their forms and sizes. They form a significant class of medieval monument and are important for the understanding of the distribution of wealth and status in the countryside. Many examples provide conditions favourable to the survival of organic remains.

The moated site at Groombridge is of particular importance because the moat survives in its entirety and remains wet throughout the year so that the archaeological potential for the survival of normally perishable artefacts and other evidence is high. The potential for the recovery of evidence both of the ground plan of the original buildings at the site and of the development of the manor is also great, especially on the northern side of the island.