Please help me decipher this poem...!?

Question

Then rose the King and moved his host by night

And ever pushed Sir Mordred, league by league,

Back to the sunset bound of Lyonesse--

A land of old upheaven from the abyss

By fire, to sink into the abyss again;

Where fragments of forgotten peoples dwelt,

And the long mountains ended in a coast

Of ever-shifting sand, and far away

The phantom circle of a moaning sea.

Answer 1

Arthur's army pushed Mordred's west to the western boundary of Lyonesse, a mythical land thought to have existed where the Bristol Channel now is.  (Look at a map of Great Britain--it separates southern Wales from Cornwall.)  Tennyson seems to assume that Lyoness rose out of the sea as a result of a volcanic eruption and had sunk back into the sea between Arthur's time and his.  He seems to have imagined it peopled by descendants of very ancient inhabitants of Britain.

Answer 2

"Then rose the King and moved his host by night

And ever pushed Sir Mordred, league by league,"

A league is an archaic measurement, usually three miles or one hour's walk.

King Arthur and his men made a night march and took Mordred and his men by surprise, forcing them into retreat.

"Back to the sunset bound of Lyonesse--"

Sunset bound = poetic way of saying western boundary.

Lyonesse was a country in Arthurian legend, but there was a lot of confusion about where exactly it was. In the oldest stories, it was actually the country of Lothian in central Scotland, but this changed with the passage of time until in the later stories, it was somewhere off the south coast of England. Tennyson's poem refers to it as a land that rose from the sea by volcanic forces and was doomed to sink again:

"A land of old upheaven from the abyss

By fire, to sink into the abyss again;"

The line:

"Where fragments of forgotten peoples dwelt,"

Probably refers to the idea that Lyonesse was a place where a few survivors of the sunken Atlantis found refuge

The rest is fairly obvious:

"And the long mountains ended in a coast

Of ever-shifting sand, and far away

The phantom circle of a moaning sea."

A long chain of mountains runs down to the coast where there are constantly shifting sand dunes and the land is encircled by an ocean whose surf you can hear.