Visited here today on a gloriously sunny Bank Holiday weekend.
Access is fairly straighforward. On the road just sough of the camp, it's possible to pull in by a marked bridleway. Follow the bridleway two fields north, then turn right. The camp is laid out in front of you, enclosed by a fence and gate.
Beware if sheep are in the enclosure (as they were today), as the gate does not meet properly, and it's not possibile to secure it shut - remember the country code?
The camp itself is as JackSprat says, quite impressive in its size. The bank and ditch are still very well defined for the most part.
A most worthwhile stop on what was a pleasant BH drive.
Rainsborough Camp is the remains of an Iron Age hill fort, located approximately 1/2 mile south of Charlton, on the borders of Northamptonshire and Oxfordshire.
It is said to have been constructed between the 4th and 6th Century BC. Although longbarrows around the site date to 3000 BC (Source: Aynho today website).
Around 150 BC, the camp was burnt to the ground and remained unnoccupied until around AD 250 when Romans settled there as an un-fortified camp.
The site is situated on the brow of a hill and it's size is impressive. Its earthworks measure over 8 feet deep in some places and up to 15 feet high. It is entirely surrounded by a shallow ditch. Inside, the camp could easily hold three football pitches - the surface area is roughly six acres.
A brook runs down the valley towards Kings Sutton at the bottom of the hill. There are two entrances, to the east and west, although the east facing entrance is much larger.
The remains of long barrows can still be identified to the north and east of the camp, although farming has destroyed much of them.
In this parish, to the south of the village, is a spacious valley called Danes-moor, or Duns-more, where, it is said that a sanguinary conflict took place between the Danes, who had in great force encamped on the heights of Rainsborough, and an army of Saxons collected to oppose their depradations. But as this is not mentioned in the Saxon Chronicle, it merely rests upon tradition. Greater credibility attaches to the account of a battle fought here between the leaders of the two contending factions for the houses of York and Lancaster..
p74 in volume 11 of 'The Beauties of England and Wales, Or, Delineations, Topographical, Historical, and Descriptive' by John Britton (and others), published 1810.
~Online at Google Books.