What Classic Marathon is
Classic Marathon is the 1994 Bungie first-person shooter, rebuilt for modern machines by the fan community using the Aleph One engine. You play as a security officer aboard the UESC Marathon, a colony ship in orbit around humanity's first interstellar colony in the Tau Ceti system. Alien forces have boarded the ship, and your job is to fight them off and protect the crew. The game runs on the original data files, so the levels, weapons and encounters are the ones from the original release, presented with optional modern touches: widescreen HUD support, 3D filtering and perspective, positional audio, and interpolation above 60 fps if you want the look to be smoother than the 1994 original. It is a linear, story-driven shooter in the classic boomer-shooter mould, with aliens as the main opposition.
Playing it with your group
Steam lists both online co-op and online PvP for Classic Marathon, so a group of friends can team up against the alien threat or fight each other. The co-op side fits the campaign structure: you work through the ship's levels as a squad, dealing with the same enemy encounters the singleplayer campaign throws at you. The PvP side gives the group a way to play against itself when you would rather not push through the story again. Since the game is free to play, getting everyone into a session is mostly a matter of installing it and connecting.
Before you start
Expect a linear campaign rather than open-ended levels: you move through the Marathon's areas in a set order, clearing rooms and progressing. The pacing and feel come from an early-90s shooter, so movement, aiming and enemy behaviour follow that era's conventions even with the optional visual and audio upgrades switched on. The engine itself is open source, released under the GNU General Public License Version 3, and the project is maintained by the fan community rather than by Bungie. If the original presentation feels too authentic, the optional settings let you soften it without changing the underlying gameplay.