In 1536, the dissolution of Tintern Abbey would have had a negative, effect on the area and those dependent on it for work shelter and sustenance. Thirty years later, in the reign of Elizabeth I, the Angidy Valley location offered a site with a local experienced workforce, (metalworking), pre-existing water-powered mill structures, ample fuel, raw materials, minerals and transportation links. An ideal location for the establishment of a company set up to improve ironworking using waterpower and charcoal-fired furnaces.
In 1566, in an effort to make Britain less reliant on foreign sources for armour and munitions, Elizabeth I granted Letters Patent to erect 'batterys' at Tintern for working Iron; to The Company of Mineral and Battery Works, (Macdonald, M, B, 1961). -The battery in this context refers to a water-powered array of hammers for crushing mineral or shaping hot iron, and not the modern electric power storage devices; although, it is an interesting coincidence.- A little later, experimentation in iron wire production for carding combs required by the burgeoning wool industry would later lead to 3 wire works being built to deal with the demand. The 'secret medieval tunnel': was part of the middle wire works, see Parr & Tuckers' 1821 schematic plan taken from the 1763 Aram survey.
I discovered a section of the Middle wireworks water management system, close to the 'secret tunnel' in 2012, APAC. Ltd WB/CCT/12.
To summarise: 1566 is not the medieval period, and the wireworks to which the 'secret tunnel' belong were recorded on a 1763 survey, (ARAM, J., 1763), and later lease surveys and ordnance survey maps until 1886.
I have spoken with the owners of the associated site I manage, and we decided to offer a more rational explanation of the new discovery based on known facts; consequently, we have to break the story of the actual discoveries, on-going at Tintern, as their context provides archaeological comparisons. All the fully illustrated APAC. Ltd reports mentioned in this account can be accessed online from Scribd or requested from Royal Commission on Ancient and Historical Monuments Wales (RCAHMW), Glamorgan Gwent Archaeological Trust Historical Environmental Record (GGAT) HER, or Gwent Archives.
None of the structures described, except for the Scheduled Monument, Tintern Furnace Site, are open to public access and all excavated structures to date have been recorded and backfilled for preservation.