SAN FRANCISCO -- In the latest season of Netflix’s “House of Cards,” there’s a major product placement for “Monument Valley,” one of the more popular smartphone games of 2014. That showcase has given a big boost to the game’s downloads, but surprisingly, ustwo, the company that made “Monument Valley,” did not have to pay a dime for the spot.

“They called us up and they said, ‘Can we use your game?’ and we said ‘Yes,’” said Ken Wong, the game’s lead designer, speaking at the Game Developer Conference here Thursday. No money was exchanged, Wong said, adding that ustwo hasn’t spent anything to market the title. Wong said his guess is that one of the show’s writers must be a “Monument Valley” fan.

“It was just a cool collaboration,” he said.

In "Monument Valley," users must guide a "silent princess" through surrealist palaces that are riddled with optical illusions. The game features gorgeous visuals along with fun yet complex puzzles. It was released last spring for $3.99, and it has since been downloaded more than 2 million times and collected numerous awards, including the Apple Design Award and the 2014 iPad Game of the Year award, it's been listed among Google Play's Best Games of 2014.

The game was a surprise hit as it came from ustwo, which does not focus on gaming. The company is a digital product studio that builds apps of all kinds for global customers, including Google, Sony and HTC. For ustwo developers, making games is typically just a fun hobby. But in this instance, “Monument Valley” managed to rake in more than $5.8 million in revenue -- and that's excluding the recent "House of Cards" boost.

Prior to its Feb. 27 appearance on the series, “Monument Valley” ranked outside the top 25 paid games on both Google Play and the Apple App Store, but about five hours after the third season of “House of Cards” debuted, the game’s rank skyrocketed.

Why five hours? Each episode is roughly an hour long, and the “Monument Valley” cameo comes in the fifth episode. Once eager “House of Cards” fans saw it, the game started climbing the rankings of both app stores, according to appFigures, a reporting platform for mobile developers. “Monument Valley” had a tremendous 2014 and received many awards, but after “House of Cards” released, it now sits in third place on the Apple App Store's list of top paid iPhone games and eighth place on the equivalent list for Google Play.

In the show, main character Frank Underwood (Kevin Spacey) is seen playing the game and later reading a pretty great description of it. He likened it to something that late Dutch artist M.C. Escher, known for visually perplexing renderings like Drawing Hands, “could only grasp at in a dream state.” Said Wong: “I told my producer to make that the description of the game.”