The River Elwy has its source a long way to the west on the flanks of Moel Seisiog south-east of Llanrwst.

The Elwy flows principally eastwards to join the Clwyd passing through few villages.

At the confluence of the Elwy and the Clwyd, the river becomes tidal and enters a narrow estuary with much wildlife before meeting the Irish Sea at Rhyl.

In order for his supply barges to reach the castle, he employed 968 men to work on what was called 'The Great Ditch'.

This was an enormous feat of engineering, compared even to today's modern standards, and it is still possible to make out the original loops and turns of the Clwyd using satellite imagery.