Which 2005 freeware DigiPen game led to Portal's creation?

The 2005 freeware DigiPen game that led to Portal’s creation was Narbacular Drop. Developed as a student project, it featured a distinctive portal system that allowed players to place linked portals on suitable surfaces and use them to navigate a dungeon.

Valve developer Robin Walker encountered the game at DigiPen’s career fair and became interested in the team’s unusual approach to movement and puzzle design. He invited the developers to present their work at Valve, where the concept impressed company president Gabe Newell and other staff.

The resulting Portal was not a direct remake. The original dungeon setting and princess protagonist were replaced by the Aperture Science Enrichment Center, Chell, and the testing framework built around GLaDOS. Valve retained and refined the central portal interaction, including the relationship between portals, gravity, and momentum.

Narbacular Drop also helps explain why Portal feels unlike a conventional teleportation game. Its puzzles treat portals as physical tools for changing routes, velocity, and perspective. Another DigiPen project, Tag: The Power of Paint, later influenced Portal 2’s mobility gels, but it was not the game that originated Portal.