A banjo enclosure comprises a central area, usually of curvilinear plan and less than 0.6ha in extent, bounded on all sides by a ditch and outer bank, a single entrance approached by double parallel ditches defining a trackway, and some kind of Paddocks may be attached to the central enclosure and/or the trackway, and in some cases the whole complex is enclosed within a compound.
Banjo enclosures are mostly recognized as cropmarks or soilmarks from aerial photographs, although a few are known as earthworks. Banjo enclosures are sometimes confused with other classes of enclosures, particularly small enclosures of roughly round or square plan. The distinctive arrangement of ditch and outer bank usually allows banjo enclosures to be recognized fairly easily where the nature of the boundary earthwork can be seen.
Specifically excluded from the definition of banjo enclosures are enclosures of various other classes which are set beside trackways and enclosures which have antennae ditches extending from their entranceways but no trackway forming an approach to the central enclosure.
Recent studies of banjo enclosures suggests that most, if not all, were settlement sites with elaborate entrances. As such, the function of banjo enclosures probably overlaps with that of several other classes of late prehistoric monument.