Susan Crossland, from Mirfield, has spent £5,000 on the gaudy construction for 10-year-old lhasa apso Archie.
The pampered pet will sit in his regal residence while Susan hosts a garden party to celebrate Prince Harry's wedding in May.
The lifelike castle is velvet-lined and hand-painted, and stands at two metres high.
Susan Crossland: The lottery winner whose win brought joy to her siblingsIt even has a throne and hot tub for Archie to enjoy.
Mother-of-four Susan and her husband Michael scooped a £1.2million Lotto jackpot in 2008.
"I love royal weddings and I love the royals. I just thought it might be a bit quirky to have something nice for Archie so I had him a Windsor Castle made and he absolutely loves it.
"It's worth it. He's my dog at the end of the day. Why not spoil your dog?
She commissioned specialist sculptor and set designers the Russell Beck Studio, from south London, to create Archie's very own palace.
A seven-strong team took 244 hours to build it using 15 litres of paint.
"We have been preparing our verandah, getting out the flags and buying lots of red, white and blue trimmings in recognition of the big day.
"Archie is obviously a big part of the family so we couldn't have him missing out.
"We love to spoil him and I couldn't believe it when I heard about a gentleman who was up for all types of projects - no matter how crazy."
"I want it to be just perfect for him and to replicate Windsor Castle as far as possible.
"Archie obviously can't go to Windsor Castle so we wanted to make him his very own castle - and we know he will love it."
Susan's lottery win was serendipitous as she took on her father's favourite sequence of numbers after he passed away - and won with them on the anniversary of his death.
This Life: Lottery millionaire Susan CrosslandShe used the money to care for her brother and two sisters, who have learning difficulties and who would have had to live apart after their parents' deaths. Instead, they have moved in to Susan's purpose-built home.
Generous Susan also plans to donate the castle for public use after the royal wedding is over.
"Although I had it done for Archie - and he does love it - I would like after the wedding to take it off him, if he'll let me, to donate it to a place where lots of other people can enjoy it - either for dogs or maybe it can go for a place when children can play in it."
Susan and Michael survived on earnings from his dairy farm job before their win, although he missed the routine of work and went back to his old role soon after.
The family faced heartache when daughter Samantha, then 30, was convicted of stealing £22,000 from a children's nursery in Dewsbury where she worked in 2015. She avoided jail after paying back the cash, which came from parents' fees.