To celebrate Commander Keen's 25-year anniversary, id Software co-founder John Romero has shared the demo that started it all for the platforming series.

Romero and company actually created a PC demo version of Super Mario Bros. 3 in 1990, which they reportedly pitched to Nintendo. However, Nintendo said no, so the team utilized the side-scrolling tech created for the demo to power Commander Keen.

Along with the video, Romero wrote on Vimeo: "This is a demo of IFD's (soon to be id Software) Super Mario 3 demo created for Nintendo on the PC. The demo was completed on September 28, 1990."

Nowadays, it's a tad easier to make Mario levels, thanks to the release of Super Mario Maker. But it's neat learning new tidbits about the NES classic. In fact, earlier this year, Shigeru Miyamoto revealed that Super Mario Bros. 3 was all a play.