Marshmallow Doom is a source port that lightly sweetens classic Doom gameplay with new additions such as dynamic gameplay-driven music, cooperative bot players, persistent player stats, enhanced blood and gore, treasure items, new difficulty levels, customizable sandbox battles, a much-improved multiplayer experience, and more.

Here are just a few of Marshmallow Doom’s exciting features:

  • Dynamic music soundtrack that morphs based on the player’s in-game experience, taking Bobby Prince’s original soundtrack to a whole new level
  • Re-balanced multiplayer experience adding modern features such as progressive weapon loadout, dropping ammo for squad mates, conserved item pickups, weapons stay, friendly fire on/off, and more
  • Bots available for cooperative or deathmatch games
  • Treasure mode helps give Doom an even more classic vibe like that of Commander Keen or Wolfenstein 3D
  • Sandbox mode allows the player (or multiple players in a network game) to build their own custom battles in any map
  • Monster upgrades can give any old map a new challenge, even repopulating Doom maps with Doom II monsters
  • New skill levels Ultra Violence 2.0 and Nightmare 2.0 utilize monster upgrading to create an entirely new standard for Doom’s difficulty
  • Inventory system allows the player to store items for future use, such as radsuits, invisibility, portable medkit, and more
  • Both Doom and Doom II IWADs can be combined so that music, sound, and graphics from both games can be used
  • Enhanced blood, gore, and epic boss deaths
  • The invisibility powerup now makes the player truly invisible to enemies, introducing a stealth tactic to help the player better survive any Doom level
  • Overplayed songs such as those from MAP01, E1M1, and intermission songs can be blacklisted, where an alternate song is randomly selected in its place
  • Player stats such as total kills and accuracy are tracked by the player profile
  • Various monster and weapon mutators
  • Most new options are quickly accessible via the player’s HUD datapad, without having to tediously dig into Doom’s options menus
  • All preferences are saved to disk for future sessions, and a separate server.cfg configuration file is available for multiplayer-specific game settings

For the full documentation, see the wiki.  (Alternate link here)