Bodelwyddan Castle re-opened on Friday with 45 new garden lodges a new lounge, dining area and entertainment facilities.
Owner Warner Leisure Hotels said the work provides capacity for an additional 10,000 guests to stay each year.
Landscaped gardens and ponds have also been created.

Fletcher Porrit, was injured by the Cleveland Police dog when it was called to an unrelated incident.
He was hurt near Farleigh Close, Billingham, at about 20:00 GMT on 3 March, the Independent Police Complaints Commission said.
Commissioner Carl Gumsley said it was "vital that we collate all the relevant information".

President Putin signed a decree to make the award to the martial arts expert, who starred in Hollywood action thrillers during the 1990s.
Seagal is friends with Mr Putin, who he has described as "one of the greatest world leaders".
Mr Putin's spokesman said Seagal was known for his warm feelings towards Russia and his status as a famous actor was a reason to make him a citizen.
Seagal made headlines when he described Russia's actions in Crimea, which it annexed in 2014, as "very reasonable". A keen guitarist, he played a concert there the same year.
In 2013 Mr Putin reportedly proposed that Seagal be made an honorary consul of Russia in California and Arizona.
The move would have made Seagal a potential intermediary between the White House and the Kremlin, Buzzfeed reported. The US refused.
Seagal, whose grandmother was from Vladivostok in Russia's far east, has made frequent trips to Russia in recent years and visited Kamchatka and Sakhalin in September.
He said during the visit that he wanted to spend several months a year in the country, Russia's Tass news agency reported.
Seagal follows other celebrities including French actor Gerard Depardieu, US mixed martial arts fighter Jeff Monson, US boxer Roy Jones and US snowboarder Vic Wild in taking Russian citizenship.
Earlier this year Seagal was given Serbian nationality after offering to set up a martial arts school in the capital Belgrade.
The action star's movie credits include Under Siege and Flight of Fury and he has also starred in his own US reality TV show, Steven Seagal: Lawman.

6 February 2017 Last updated at 09:46 GMT
British player Kyle Edmund was beating Denis Shapovalov from Canada, when frustration got the better of him.
The 17-year-old Canadian whacked the tennis ball in anger, only to hit the umpire in the eye!
As a result the match could no longer continue, meaning Great Britain won by default.
Kyle will now go on to face off against France in April.
Shapovalov apologised to the umpire after the match, and his coach hopes he will learn a lesson from it.

The 18 sketches are by artist Donald McGill who produced more than 12,000 original artworks for the postcard industry between 1904 and 1962.
Along with a pun-filled caption, the pencil sketches include stamped approval by the censorship board.
They sold for between £55 and £220 and fetched about £1,200 in total, auctioneers Moore Allen said.
Described as the "king of the saucy postcard", McGill produced thousands of artworks and an estimated 200 million postcards were printed and sold.
Often with a double, risqué meaning, one of his sketches shows a buxom blonde flashing her bloomers as she waters a flowerbed with a passing man saying: "I like her display of pinks, Fred!"
Another features a red-faced policeman telling a curvy blonde driver with a broken taillight: "You've got no light behind, so I'm afraid I'll have to pinch you."
But in the 1950s, McGill fell foul of a crackdown on the saucy postcard industry and was prosecuted and fined under the 1857 Obscene Publications Act.
Many postcards were destroyed by their owners during the government crackdown.
One hundred new jobs have been created following a £7m investment at a Grade II-listed hotel in Denbighshire.
[NEXT_CONCEPT]
A witness appeal has been launched after a 10-year-old boy was bitten by a police dog.
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US actor Steven Seagal has been given Russian citizenship, the Kremlin says.
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Great Britain are through to the Davis Cup quarter-finals, after a Canadian player accidentally hit the umpire with a tennis ball.
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Original bawdy sketches the basis of thousands of "saucy seaside" postcards have been auctioned in Gloucestershire.