Doom II for Game Boy Advance

Doom II for the Game Boy Advance was developed by Torus Games of Australia and published by Activision. This version of the game does not run on a port of the original Doom engine, but instead runs on a custom engine shared with several other first-person shooter games called the.

Differences from Doom for GBA
Some of the differences between Doom for Game Boy Advance and its successor, besides the maps, include the fact that all maps and monsters are present, including the Wolfenstein 3D secret maps and Wolfenstein SS enemy. All items are also present, including the previously absent light amplification visor and the blur artifact. The demon has its original death sound, health potions and armor bonuses give only 1% versus 2%, and the original PC status bar is used, albeit rescaled for the 480x320 resolution.

Since the original Doom engine was not used for this game, all content and mechanics had to be converted to the Southpaw Engine. In terms of content, the game is nearly identical to the PC version, with only very minor changes. However, conversion of the game play and rendering functions to the new engine resulted in a number of visual and mechanical inconsistencies from the original game; some are purely cosmetic, but many have a significant&mdash;and often detrimental&mdash;effect on game play.

Censorship
Like its predecessor, Doom II for the Game Boy Advance is heavily censored in order to obtain a "Teen" rating from the, as the first party console manufacturer discouraged creation of mature-rated games for the platform, focusing its marketing heavily on the  demographic. As a result, monster blood is green instead of red, and most instances of extreme gore, such as gibs, were removed. Additionally, if the player is damaged, a bullet puff is produced instead of a blood splatter. The status bar face is also altered so that the blood is brown instead of red. Strangely, the super shotgun's shells have also been recolored, even though they did not depict blood or gore.

Graphics
The color palette used is not identical to the color palette used in the original Doom games, resulting in some objects or surfaces suffering from palettization issues, especially under certain lighting/colormap conditions (such as being under the influence of a Berserk Pack). Browns and tans seem most affected. The pain elemental displays this more noticeably than other examples; its body appears slightly splotchy even under "normal" lighting. The first "episode" skymap also illustrates this.

Due primarily to the pervasive darkness of the first generation Game Boy Advance's display, the graphics are significantly brighter than their PC counterparts.

Maps
Most maps are modified to some degree, mostly to control the amount of geometry visible from certain locations. Examples include extra walls added in MAP03: The Gantlet and MAP06: The Crusher which block lines of sight across large areas. In addition, the maps are based on their v1.666 builds as opposed to the v1.9 IWAD versions - for example, silver bars are still present on the left cubby hole near the red key in MAP02: Underhalls, and an extra platform added later to MAP11: 'O' of Destruction to make the jump into the final area easier is not present.

MAP15: Industrial Zone and MAP24: The Chasm are both split into two maps each to avoid slowdowns and memory constraints. However, the scoring on these maps was not adjusted to reflect the removal of the enemies, secrets, or items relocated to the corresponding "second" map, making a score of 100%/100%/100% on these levels impossible. Whether the "removed" entities are still present in the map, or whether these maps have a special hard-coded scoring exception, is unclear.

The Wolfenstein secret maps and Wolfenstein SS enemy are present, but all imagery and  paintings have been replaced by iconography from, presumably to avoid having the game confiscated in Germany and other jurisdictions with restrictions on depictions of Nazi and  symbols.

Mechanics

 * A message is displayed on-screen when the player finds a secret area.
 * Doomguy's status bar face changes at different health values than in the original game (about 5 - 10 points lower health for each transition).
 * Like the first GBA Doom, "Ultra-Violence" was renamed to "Nightmare!"; true "Nightmare!" was removed.
 * Vertical auto-aim is not reliable; the game will usually not aim for barrels on a lower altitude than the player, and hitscan shots fired at floating/flying enemies at close range will sometimes be fired above or through the monster instead of at the monster.
 * Player movement when strafing or backpedaling is much slower relative to other versions of Doom, making it more difficult to avoid projectile attacks or sudden ambushes.
 * Health items such as medikits and stimpacks are now erroneously counted towards the "Items" tally at the end of the map, making a score of 100% more difficult to achieve (since health items can only be obtained if the player is not at full health).
 * The damage mitigated by armor seems to be no more than 30%, regardless of which armor type the player has.

General changes
Enemies are drawn slightly larger relative to the environment than in other versions of the game; however, their bounding boxes are not scaled to the same degree. This makes shots tend to "miss" when they should appear to hit, if the shot was aimed at the periphery of a monster's visual representation.

The player is now allowed to walk underneath flying enemies, or jump over the heads of monsters below - this was possible in Doom engine games such as Heretic and Hexen, but was not allowed in vanilla Doom. Monster corpses will however obstruct the motion of a door, causing it to reverse if it tries to close on a corpse (as opposed to crushing or destroying the corpse). Since monster corpses disappear after a few seconds, this is only a temporary issue.

Enemy AI will shut off at times, presumably to avoid slowdown due to heavy processing. The decision by the engine to shut off any given enemy's AI seems to be based upon a combination of distance and/or number of sectors between the monster and the player, and NOT based exclusively (if at all) on whether or not the player is within line of sight. Such a monster can sometimes even be shot and damaged by the player and remain "dormant" (an example is the Imps across the water in the first outdoor area in The Gantlet).

Enemies killed by traps or friendly fire from monster infighting are erroneously not counted towards the "kills" tally at the end of the map. Only monsters killed directly by the player appear to be counted.

Specific monster changes

 * Arch-vile: The flame attack's sounds are missing and the explosion sound (DSBAREXP) is played when the attack is initiated rather than when it finishes. Also, the flames appear lower in the player's view, and damage dealt is dramatically lower.
 * Commando: His chaingun produces the same gunshot noise as the player's, rather than using the shotgun's sound effect.
 * Lost soul: The bounding box of the lost soul's attack is much larger than it should be, and causes the player to receive damage if the player is within several feet of the lost soul in any direction when it attacks (sometimes even if the player is behind cover or around a corner) and not just on a direct hit. Not only does this result in easier hits against the player, but it also makes using the chainsaw impractical, as the player will suffer damage from the lost soul's attack long before the chainsaw can make contact with the lost soul.
 * Mancubus: He turns his body roughly 45 degrees from the player during his first and second attacks, instead of simply angling the path of his projectiles, which usually causes his projectiles to miss badly. His third attack is performed more or less normally.
 * Revenant: Its missiles do not smoke when homing, and are fired from the revenant's waist (approximately 32 units high, like all other missiles). Non-homing missiles are rarely if ever fired.

Music
The music tracks "Between Levels" (MAP04) and "Getting too tense" (MAP28) were dropped, most likely due to cart space. In their place was "Into Sandy's City" and "Evil Incarnate", the music to MAP09 and MAP31, respectively.

Weapons and ammo

 * Dropped weapons and items give full ammo, instead of half.
 * The box of rockets gives ten rockets rather than five.
 * "I'm Too Young To Die" skill does not give double ammunition; it only reduces damage.
 * Weapon damage dealt to other players during deathmatch is reduced by 75%, making deathmatches much more protracted than usual.
 * Chainsaw: It pushes enemies away from the player, instead of simply dragging the player into the enemy; this makes using the chainsaw risky and difficult, as the enemy can be pushed out of the chainsaw's area of effect, allowing the enemy a chance to counterattack.
 * Shotgun: Its behavior was changed so that it fires in a conical pattern instead of a fan pattern; though this behavior is more "realistic", it deviates from the original design of the weapon and affects its usage significantly, as "grazing" indirect hits or hits against multiple enemies are much more difficult due to the substantially lessened horizontal dispersal.
 * Super shotgun: It reloads faster; also, its sound effects are not synchronised with the animation, resulting in the animation playing longer than the sound effects.
 * Chaingun: It no longer fires in bursts of 2; instead, it fires only 1 bullet at a time when used. It also has a noticeably higher rate of fire. Both the pistol and chaingun no longer fire with perfect accuracy when single shots are fired; instead, the bullets are fired with the same deviation as though fired during a sustained burst. This makes "sniping" or chaingun tapping impossible.
 * Plasma gun: It no longer plays the "recoil" animation that normally plays after releasing the attack button; this makes consecutive usage of the weapon (such as single shots or short bursts) faster.
 * BFG 9000: Its tracer attack uses the BFG plasma ball explosion sprites, and the explosion sound used for the BFG plasma balls impact is the standard explosion sound rather than the normal unique effect.

Bugs
The armors system in this game is somewhat broken; armor only provides protection in the map it was collected in, even though the numerical value of the armor carries over from one map to another. If the player enters a map with any armor percentage above 0%, the game will ignore the player's armor value, causing the player to suffer full damage from attacks. This happens until an item which affects armor value is picked up (either armor, megaarmor, a megasphere, or an armor bonus will work); this will "reboot" the armor system to properly protect the player again. If such an item is unavailable (or if the player can't collect the item because their current armor value is higher than what the item offers), then the player is, for all intents and purposes, forced to play with no armor, even if the player's actual armor value indicates the player should be protected. This was confirmed as a bug by publisher Activision.