Like me you have probably seen romantic paintings of Harlech Castle with sailing ships moored at its base. Like me you have probably told a passing tourist about the sea and the ships supplying the castle. Artistic licence has taken a liberty and we're all talking codswallop.

The official guide (1974) says ... "On arrival it is hard to realize that the forbidding crag, when the fortress was built upon it, stood on the verge of Tremadog Bay with a harbour at its base. Since then the ocean has withdrawn, leaving a half-mile strip of marsh and dune where caravanners reside by the season and championships are played on the links ...."

I know climate change is happening faster than we expected but surely this is taking it a bit far? Could the sea have been deep enough for ships, all the way to the base of the rocks? Has it dropped or retreated that much in just over 700 years?

Just to add to the confusion there's a pub called the Ship Aground on the road to Harlech, was it the site of a shipwreck?

Not so. The castle was indeed supplied by sea but the final stretch was by canal (y gamlas) running from the sleepy village of Ynys. As for the pub this name comes from Captain William Jones who bought land here in 1805 and said of his new home "this will be my ship aground".

Ynys today looks an unlikely starting point for a canal with a lot more mudflats than water. But what the eye does not see is the original course of the river. There was a great storm in 1816 and instead of flowing past Ynys the river drifts past Portmeirion, the Italianate village.

There is a lock at Ynys alongside a tall building called The Warehouse (Y Warws) but this is not a 2-way canal lock for boats, it is a 1-way drainage lock. Since 1806 the canal has been subsumed within the extensive drainage systems that created agricultural land from what was known as the Great Marsh.

Anyone who tells you the sea came up to the rocks 700 years ago is talking codswallop. It was the Great Marsh and the canal that lapped the base of the crag on which Harlech Castle was built.