As a first attempt!

It is of thee I speak, fairest Gwen, fairest Gwen
From the Usk to Snowdon's peak, fairest Gwen
(Failed translation, but retains idea of from South to North)
And to yonder castle's keep (Stately heap)
You must turn to find your sleep
Where the fam'ly will you keep
(So) venture forth, fairest Gwen

See ahead a welcome peak, Fairest Gwen, Fairest Gwen
Where your journey you can break (sight rhyme only), fairest Gwen.
And with you then by my side,
To go on we will decide
And the castle rest inside,
(Now) Venture forth, fairest Gwen

Now, that castle is mine own, fairest Gwen, fairest Gwen.
Once your willingness you've shown, fairest Gwen.
As the wife of Castle Crohn (massive assumption, but avoiding nasty rhymes & misplaced accents)
You'll be welcomed as our own,
And belong to me alone. Fairest Gwen.


I don't know the tune, but what I've written (based on my reading of the meter of the Welsh version) seems almost to scan to "Let us haste to Kelvin Grove"