Castell y Fflint

The castle stands on the edge of Wales by the Dfrdwy Estuary in the ancient cantref of Tegeingl. It is known by locals as the last castle because it stands at the end of what was the known world, between Wales and the Lost Lands across the water in the perilous realm of Cilgwrle (The Wirral), where, amongst other perils, the Green Knight dwelt.
The current castle was in fact built by Edward I to oppress the Welsh. However, the fortress was built without appropriate rituals and is classed by bards as a Nwyfgaer (half-castle). It is not only on the edge of Cymru but the edge of worlds. It was built in what was seen as defiance of the river goddess Aefren, and has been the subject of consistent and unexplained erosion forcing diversion of the river. It has been destroyed several times in multiple ways.
It is known to be the subject of a melltith (curse). This is a generalized spell “representing a concept” – in this case treachery/betrayal. Consequently the land, plants, animals and people will tend towards untrustworthiness at this place. Friends fall out, plants poison, building foundations erode and pets turn on masters.
At this place in 1399, Richard II was turned on by his own cousin, Henry Bolingbroke (Henry IV). Richard’s endlessly loyal greyhound, Mathe, deserted his master and ran to Henry.
Since the building of the castle, the poisonous water hemlock has spread across the surrounding land.