Just a few minutes outside of downtown Phoenix, on the north face of South Mountain is a house unlike anything in the Valley.

Constructed from desert rock, stone, boulders, misshapen brick and any other recycled or discarded materials that could be scavenged -- it has been there since long before Phoenix was the Valley we know today.

Many call it “The Mystery Castle,” but to Mary Lou Gulley it was simply called “home” -- a promise left to her by her late father.

If you build it

The home was built by Boyce Gulley -- a man who had a lot more love for his daughter, Mary Lou, than experience actually building homes.

Marshall Trimble, Arizona's official state historian, who first saw the Mystery Castle in the 1950s, says the story of why the “castle” came to be is a “very touching” one.

Gulley lived in Seattle and would spend time at the beach with Mary Lou playing in the sand. She was his little princess and he would always build her sand castles. But as is true with all sand castles, the water would eventually wash them away.

Mary Lou would cry, so her father promised her that one day he’d build her a real castle.

However, Trimble says, Gulley just disappeared one day in 1927.

He left his family behind for Arizona.

“He was diagnosed with tuberculosis and that was a death sentence back in the 1920s and earlier," Trimble said. “People came here to Arizona hoping to be cured and some were and some weren’t, and he came to Arizona.”

Gulley acquired a large piece of land right at the foothills of South Mountain. He spent the rest of his life building a castle for his little princess.

He died in 1945 -- before he could welcome his family to their new home in the desert.

A princess and her castle

Mary Lou Gulley was an adult by the time she moved into her “Mystery Castle” with her mother.

“[Boyce] wrote her a letter just before [he died] and he said ‘I left you your castle,’” Trimble said.

But it wasn’t always called a “mystery.” The name “Mystery Castle” stuck thanks to a 1948 issue of Life magazine.

And at some point, possibly thanks to the buzz created by the magazine, Mary Lou started sharing the “mystery” with visitors and all those who wished to see it.

Juankarlo Gastelum, who grew up in and around the home, has been a tour guide at the castle for nearly a decade and his father a caretaker on the grounds for over 40 years.

Gastelum started in what he recalls -- with a smile -- a “trial by fire.”

“Mary Lou’s definitely a character, if she hollered at you to do something you definitely did it,” he said. “She recruited me and my cousin, took us through the house once and then initiated us as tour guides the same day.”

Gastelum described Mary Lou as a cowgirl “through and through,” who could be a little bossy, but was always kindhearted and loved to tell stories.

“She would keep people here for hours -- just sit them down on the couch and talk away,” he said.

Mary Lou lived in her castle year-round for almost 65 years of her life -- living, for much of it, without many of the amenities we enjoy on a daily basis.

“We tend to take things for granted that we have everywhere else like running water that they went 30-plus years without,” Gastelum said. “It’s just nuts, you know. It’s a different lifestyle entirely.”

And what would she do during those hot Valley summers?

“Hope for the best," Gastelum said, "Open all the windows and doors and pray for a breeze.”

The inside of a desert castle

PHOTOS: Inside the 'Mystery Castle' in Phoenix

“I just love this place,” Gastelum said. “It’s hard to find places that are very indicative of the area that you live in and this is just a nice, weird, quirky little place and I love it.”

The house consists of 18 rooms full of priceless antiques, fireplaces, including one carved to look like the north rim of the Grand Canyon, and animals -- lots and lots of animals -- painted, carved, sculpted and stuffed. The Gulleys loved animals.

Each room is also full of its own little stories -- including a trap door now guarded by a metal knight and alligator.

It was a trap door Mary Lou and her mother weren’t allowed to open. But according to Trimble, with care and “a lot of apprehension,” the door was eventually lifted.

Inside, they found money, gold, the deed to the house, old memories and everything else Boyce had saved for them.

“It took me a solid maybe two or three years before I was able to put everything together coherently and even up to today I’m still learning, “ Gastelum said. “There’s just a lot of stuff here.”

From the deck you can see the sprawling skyscrapers of downtown Phoenix. And down below the “purgatory” room sits between the chapel and bar -- which has a horse fly net centerpiece donated by the one and only John Wayne.

One of the last rooms built in the house was the guest room -- which Gastelum calls his favorite and the most beautiful room in the house.

“A lot of history in just that one room,” he said. “This guy [Boyce] in building the house never really knew exactly what he was doing and you can just kind of see the progression throughout the house and being the final room it’s one of the more put-together rooms.”

The room was built around a living saguaro cactus -- it stands as a skeleton of its former self today.

The only room Mary Lou never shared with anyone: Her bedroom, which sits atop her castle.

Keeping the mystery alive

Mary Lou died in 2010.

“Mary Lou had everything so well set up that by the time that she did pass away not a whole lot changed,” Gastelum said. “We miss her definitely. It was definitely weird getting used to the place without having her running around the house.”

Although the home has been placed on the Phoenix Historic Property Register and named one of the city’s “Points of Pride,” it remains “privately owned.” Mary Lou left her castle to “The Mystery Castle Foundation” a nonprofit organization that maintains the property and continues Mary Lou's legacy of offering tours.

“The best thing about being a tour guide here is probably the people,” Gastelum said. “You meet a lot of interesting people, everyone always has something to share, they have stories. Sometimes they know more about things than you do.”

The Mystery Castle is open October through May, Thursday through Sunday, 11 a.m. to 4 p.m. -- 3:30 p.m. is the last tour.

Mary Lou’s fairy tale may have come to an end, but it's clear the wonder, the mystery and the intrigue of her castle will live on in its guests.