Geoff Keighly’s App Store book The Final Hours of Portal 2 includes some interesting tidbits, including the fact that it was originally a prequel set in the 50s that included none of the familiar characters people have come to love, or portals. What’s most fascinating, however, is the revelation of its single-player nature.

Portal 2 will probably be Valve’s last game with an isolated single-player experience,” wrote Keighly, relating a comment made by Valve founder Gabe Newell. “What this all means is something Newell is still trying to figure out.”

Whether this means more cooperative play or a greater focus on multiplayer titles like Left 4 Dead and Team Fortress 2 remains to be seen.