Street Plan comes from A Popular Guide to Caerleon by "ISCA"
and Atwood Thorne, published in 1928 by Western Mail Ltd, Cardiff. Atwood Thorne,
MB., was the owner of "The Mount" now known as "The
Mynde". Visitors were then able, on application, to see the
castle grounds and climb the Mound. The doctor proudly boasted
"Many of the exhibits in Caerleon museum were obtained from
these grounds". He also mentioned an old tradition that an
underground passage ran from the Mound to Newport in one direction
and Caerwent in another. He added that this was "doubtful",
but that "it is hoped that it will be investigated shortly".
Unfortunately no investigation was made following this tantalising
remark, but you can read how in the 1800s a tunnel was dug into
the mound to investigate its origin by following this