Portal (game)

Portal is the original game of the Portal series. It is a single-player puzzle-based video game developed by Valve and released as part of The Orange Box on October 9, 2007 for Microsoft Windows and Xbox 360; the PlayStation 3 version released at the later date of December 11, 2007. Portal was also re-released as part of the launch of Mac OS X support for Steam on May 12, 2010; to celebrate it was given away for free until May 24, 2010 to Steam users on a promotional page on the Steam website.

Development
Portal is Valve's professionally-developed spiritual successor to the freeware Narbacular Drop, the 2005 independent game released by students of the DigiPen; the original Drop team were employed at Valve. Certain elements, like the orange/blue system of identifying the two different portal ends a player can have open at a time (one connecting to the other), have been retained. The key difference in the signature portal mechanic between the two games is that in Narbacular Drop the player can place a portal on a wall visible through another portal, whereas in Portal, the HPD cannot fire a portal shot through a portal; however, the HPD can fire a portal shot while the player is standing in a portal.