The origin of the name "Rhyl" is not fully known.

However, the name appears in old documents variously as Hyll (1506), Hull (1508), [Leidiart] yr Hyll (1597), Rhil (1706), Rhûl (1749), Rhul (1773) Rhyll (1830), and Rhyl (1840)[2], all of which are variations (and some anglicizations) of an uncertain original form. One suggestion is that Y Rhyl derives from (Yr) Hyll, meaning "ugly" ; however the softening of final ll to l would be exceptional as would the blunt epithet as a placename. Other unsubstantiated and improbable suggestions have been made that it might derive from the similar sounding Yr Hill (as in "The Hill") or Yr Heol "(The Street)".

http://en.wikipedia.org/wiki/Rhyl

So it's really Hull?