The coastline along here is dotted with World War II pillboxes and that is initially what we thought it was. However, even the bit you can see above the sand is quite a size and if it was a pillbox it held fairly big guns. Thanks initially to the urbanrim website I can confirm that it is military in origin and from the WW II era, but it is a toppled observation tower.
Originally a small settlement, Ringborough was lost to the sea some years ago, with only Ringbrough Farm remaining. Originally built in the 1770's, the farm was selected as the location for an artillery battery during WW II. Ringbrough Farm was demolished in 2011 when it was threatened by the erosion of the cliff. In 1833 the farm buildings stood just over 300 yards from the cliff edge. The WW II battery included three gun emplacements, obervation tower, amunition store, searchlights, engine rooms and various other ancillary buildings. The Historic England (Pastscape) website provides detail about the battery and its functions.