This rather wonderful set of interlocking circular stone walls, now largely turf-covered, is the remains of a Romano-British village of first century AD. It was a courtyard settlement – unique to Cornwall, where open yards were included between roundhouses and even within the bigger ones. More here.
It replaced an Iron Age village, with bits dating back a further 500 years, including part of the underground structures ( a fogou, from Cornish for cave). The oldest underground part is a circular corbelled stone room.
Later a 20m long passage was linked to it.