Park somewhere near the large red brick pumping house (you can't miss it) and walk down the narrow lane which runs alongside it. (Don't try to drive down the lane - too narrow). When you come around the other side of the building you will see a path which takes you straight into the cliff fort. This is the only prehistoric site I have been to with a football pitch / goals in the middle of it!! Great views of the Severn Bridge.
It is almost impossible to realise the extent to which the coast-line must have altered. According to tradition, a long spit of land once ran out from Sudbrook Point in a south-westerly direction, extending as far as the Denny, a rocky islet now lying in mid-channel at a distance of over four miles from Sudbrook.
Sudbrook fort's certainly been nibbled away at by the Severn over the years. And there's a lot of mud about. It's a long way though!
In the 17th C. Camden described the erosion rather elegantly: The Church whereof, called Trinity Chappell, standeth so neare the sea, that the vicinity of so tyrannous a neighbour hath spoiled it of halfe the church-yarde, as it hath done also of an old fortification lying thereby, which was compassed with a triple ditch and three rampiers, as high as an ordinary house, cast in forme of a bowe, the string wherof is the sea-cliffe."
From AE Lawson Lowe's article on the camp in Archaeologia Cambrensis (Jan 1886).
This is taken from Coxe's 1801 'An Historical Tour in Monmouthshire':
To the west of the new passage inn, near the ruins of Sudbrook or Trinity Chapel, are remains of an entrenchment, which are usually supposed to be Roman; they occupy a flat surface on the edge of a perpendicular cliff, and are nearly in the form of a stretched bow, whose cord is the sea coast. The entrenchment is formed by a triple rampart of earth, and two ditches; the two exterior ramparts are low, and in many places destroyed; the interior is in greater preservation, and not less than twenty feet in height [...]
It is generally imagined that this entrenchment, in its present state, is not perfect, and that half of it has been destroyed by the sea, which has likewise carried away part of the church-yard. It is likewise by many supposed to have been a maritime fortress, erected by the Romans to cover the landing of their troops, adn their first station in Siluria; an opinion grounded on the erroneous description [as a square] of Harris, and on the discovery of a single coin struck by the city of Elaia in honour of the Emperor Severus. For notwithstanding repeated enquiries among the farmers and labourers of the vicinity, I could not learn that any coins or Roman antiquities had been found within the memory of the present generation. It has been also attributed to the British, Saxons, and Danes; but was occupied, if not constructed by Harold during his invasion of Gwent.
The ruins of a 12th century chapel lie among the ramparts to the south east, and Coxe mentions "Within the memory of several persons now living, divine service was performed therein; and a labourer whom I met on the spot, assisted forty years ago as pall-bearer, and pointed out the half of a dilapidated grave stone, under which the corpse was interred."