Here's what the SMR has to say about it:

The monument includes the earthwork, standing structural and buried remains of
a large multivallate hillfort, known as The Walls. It occupies a slightly
elevated position, in an area of undulating land, within an angled,
steep-sided bend of the Stratford Brook, a tributary of the River Worfe. The
hillfort is thought to lie close to the south eastern margin of the pre-Roman
Iron Age tribal territory of the Cornovii.
The Walls hillfort is roughly D-shaped in plan. Its overall dimensions are
about 340m north-south by 620m north west-south east. The defensive circuit
encloses an area of approximately 9.5ha. Its size indicates that it was
occupied by a very large community where centralised economic and social
activities were practiced, including the storage and redistribution of food
and the performing of ceremonies. The interior of the hillfort is defined
by a single rampart with steep external and internal faces. The back of
the rampart has been reduced in height by ploughing along much of the
southern side. An outer rampart was constructed on the western side of the
hillfort. This also has a steep outer face and is separated from the inner
rampart by a rock-cut ditch. Along the southern and eastern parts of the
circuit the rampart is bounded externally by natural cliffs formed by the
Stratford Brook. These cliffs formed additional lines of defence, and in
places they have been quarried in order to enhance their defensive
appearance. Around the northern and western parts of the circuit the rampart
is bounded by a broad external ditch, which has been largely infilled and is
now mainly visible as a shallow depression. It will, however, survive well as
a buried feature. The original entrance passage into the interior of the
hillfort lies to the west of the mid-point on the northern side. To the
east of the entrance, the external ditch has been recut and partially
modified by modern drainage channels. Along parts of the inner and outer
faces of the recut ditch, revetment walls are visible. They are built of
roughly coursed sandstone blocks and stand to a height of 0.7m. A wall
faced in a similar manner, with a core of river cobbles and sandstone
blocks, was constructed on top of the rampart which defines the interior.
The wall, which now survives as discontinous lengths and is partly
embanked, is between 2.3m and 3.5m wide and stands up to 1m in height.
This wall, or breastwork, was noted by JB Blakeway, an antiquarian, in
the early 19th century. All these walls are considered to be contemporary
with the occupation of the hillfort and probably gave it its name.