In Extremis

Overview

In Extremis (Latin for "in grave or extreme circumstances") begins with the player's character bailing out of their malfunctioning spacecraft, and taking refuge in the mysterious vessel they were inspecting.  Aliens have infested the ship and killed the crew.  The player must fight through 28 levels to reach the bottom of the ship and kill the Queen alien to win. 
 

Gameplay

In Extremis uses standard first-person-shooter controls to navigate the ship's levels.  Decks are laid out on a grid, similar to the design of Wolfenstein 3D. Unlike Wolfenstein, Extremis makes use of 90 and 45-degree angled walls.  Doors lock off some areas of the levels, and will automatically open as the player (if authorized) approaches. 
 
A basic inventory system is present, and the player can collect health syringes, air tanks, and batteries for later use.  Two syringes can be carried at any time, and heal any damage amount to full.  Batteries power a motion tracker and night-vision goggles, and must be replaced when exhausted.  All supplies are limited, and require some consideration toward conservation and management.  Guns themselves cannot be reloaded or recharged, and must be replaced with a new weapon when spent.  Unlike items, the player can only carry one weapon at a time, and cannot hold one in reserve. 
 
Oxygen is a special consideration, and is tracked by a green bar on the HUD.  The player's oxygen reserves are always draining, and can only be replaced by picking up limited air tanks.  The player can hold one air tank in reserve, and swap to it when the current one runs out.  If the player runs out of air, they will die.
 

Items are found in lockers placed around the levels.  These lockers are unlocked using codes manually punched in by the player with a keyboard interface.  Codes are learned by finding command cards, which display the locker/door codes for the current and upcoming level when inserted into terminals also scattered around the rooms.  Elevators require a different set of cards to access new levels, usually found in lockers.  The player can only hold three cards at a time, so they will lose access to previous levels as they pick up new elevator cards. 
 
Various alien types will be encountered throughout the ship.  Almost all of them use melee attacks, and any with ranged attacks (like spitting) are fairly weak.  Aliens use basic A.I. to charge and attack the player, but will also retreat around corners if the distance to the player is too far - forcing the player to go after them and risk being attacked.  Aliens killed at close range will leave dripping green goo on the player's helmet. 
 
While most levels use pre-placed aliens, some levels spawn them from "hives" (or passages from deeper in the ship?).  These levels send a steady stream of monsters after the player, until the player can pick up a bomb and place it at the mouth of the hive.  After placing the bomb, the player has ten seconds to evacuate the room before it detonates, closes the hive, and stops the rush of aliens. 
 

Graphics

In Extremis features textured walls, floors, and ceilings, similar to other pre-Doom, post-Wolfenstein engines (like Terminator: Rampage or Operation Body Count).  Floor and ceiling textures could be turned off to improve performance on lower-spec machines.  Colored fades are used to simulate walking under amber warning lights, or fading to white when running out of air.  The player does move slowly, but it is unclear whether this is a limitation of the engine, or part of the game design (replicating a clunky space suit). 
 
Players cannot free-look, but can crouch.  This is essential to hit aliens crawling along the floor.  The player cannot jump, but can be knocked into the air by charging attackers.
 

Criticism 

The intro movie notoriously uses audio clips ripped from the movie Alien.  Some weapon noises are the sound of photon torpedoes from Star Trek.  One particular alien type has a design that strongly copies H.R. Giger's Alien. 
 
The game does not support saves, and instead gives a password roughly every three levels.  This password is fixed, and does not store the player's own inventory and ammo level when restoring a game. 
 

Copy Protection

The player cannot progress past the first level without cards from inside the lockers on that level.  The code to these lockers is buried within the backstory printed in the manual.