You need only ponder the preponderance of castles to realise that this pleasantly rural county was once a wild frontier. The Norman marcher lords kept stonemasons extremely busy, erecting mighty fortifications to keep the unruly Welsh at bay. Despite this stone line marking out a very clear border along the Rivers Monnow and Wye, the 1543 second Act of Union left Monmouthshire in a kind of jurisdictional limbo between England and Wales. This legal ambiguity wasn't put to rest until 1974 when Monmouthshire was definitively confirmed as part of Wales.
The River Wye, Britain's fifth-longest, flows from the mountains of Mid-Wales, tootles its way into England and then returns to the middle ground – forming the border of the two countries – before emptying into the River Severn below Chepstow. Much of it is designated an area of outstanding natural beauty (www.wyevalleyaonb.org.uk), famous for its limestone gorges and dense broadleaved woodland.