official add-ons being optional. The PlayStation version contains abridged versions of Units 1, 3, 6, 7, 8, and 10 of the PC version, redesigned to meet the console's technical limitations. For example, many short airlock-like corridors were added to maps to provide loading pauses inside what were contiguous areas in the PC version. In addition, part of the first mission of the N64 port is used as a prologue. Some enemy types were removed and two new enemies was added: the Arachnid, a human-spider cyborg with twin railgun arms, and the Guardian, a bipedal boss enemy. Saving the game is only possible between levels and at mid-level checkpoints where the game loads, while in the PC version the game could be saved and loaded at any time. The game supports the PlayStation Mouse peripheral to provide a greater parity with the PC version's gameplay. The music used in this port is a combination of the Quake II original music score and tracks from the PC version's mission packs, while the opening and closing cut-scenes are taken from the Ground Zero expansion pack. The PlayStation version uses a new engine developed by HammerHead for their future PlayStation projects and runs at a 512x240 resolution at 30 frames per second. The developer was keen to retain a visual parity with the PC version and avoid tricks such as the use of environmental fog. Colored lights for levels and enemies, and yellow highlights for gunfire and explosions, are carried across from the PC version, with the addition of lens flare effects located around the light sources on the original lightmaps. There is no skybox; instead, a flat Gouraud-textured purple "sky" is drawn across the ceiling. The game uses particles to render blood, debris, and rail gun beams analogously to the PC version. There is also a split-screen multiplayer mode for two to four players (a four player game is possible using the PlayStation's Multi-tap). The only available player avatar is a modified version of the male player avatar from the PC version, the most noticeable difference being the addition of a helmet. Players can only customize the color of their avatar's armor and change their name. The twelve multiplayer levels featured are unique to the PlayStation version, with none of the PC multiplayer maps being carried over. The Nintendo 64 version has completely different single player levels and multiplayer maps, and features multiplayer