1. Introduction
Commander Keen 5: The Armageddon Machine is my personal favourite game in the Commander Keen series. Although essays should not be stimulated by subjective opinions, my preference for the title is stimulated by interesting design choices. In this essay, I wish to discuss game- and level- design with regards to planes of vulnerability in Commander Keen 5. These planes are the horizontal, vertical, and diagonal directions in which the players traverse, whilst vulnerability refers to lethal encounters with enemies or obstacles.
id Software needs no introduction; the genius of John Carmack's engine development is known today for Wolfenstein 3D, Doom, and Quake. His initial success was not the engines for these games, but his Adaptive Tile Refresh technique implemented in the Commander Keen series (see videos below). id Software developed Commander Keen in two trilogies (Invasion of the Vorticons and Goodbye, Galaxy!). A third trilogy was planned but cancelled due to the development of id Software's next groundbreaking game, Wolfenstein 3D.
Video 1: The History of id Software [development up to Commander Keen]
Video 2: The History of id Software [departure from Commander Keen]
The release and development of the Goodbye, Galaxy! were troubled with development and publishing problems. Commander Keen 6: Aliens Ate My Babysitter did not follow the plot of the trilogy, and it was developed before Commander Keen 5. Tom Hall, who designed levels for all the Commander Keen games along with John Romero, comments:
Deciding on the tilted perspective made things look really cool, but the levels look a lot longer to make. I think the overall art design was a lot cooler, though [...] I had done most of the art in the original Keen trilogy. With Adrian [Carmack] working on this new set of Keen, his skills honed over many games, the art was looking awesome. We did Keen 4, then Keen 6 (Aliens Ate My Baby Sitter), and then Keen 5. We did Keen 5 in one month. That was an amazing amount of work, but it's probably my favorite [sic] Keen [...].
(3D Realms S.A.)
As stated by Hall, the second Commander Keen trilogy vastly improved upon the game graphics of the original trilogy: a tilted perspective, along with detailed artwork, enriched the playing experience. Instead of a solid background, the backgrounds for the second trilogy comprised of intricate tiles. The design of the Commander Keen series is daring, but especially the second trilogy. Although the Commander Keen series is accessible (to children in particular), certain designs for both trilogies fringed on the absurd! Although the Dopefish is now a popular reference (as an easter egg) in many contemporary video games, other game enemies are more obscure in an extra-terrestrial setting, including the character of Princess Lindsey, a levitating NPC whose magic wand resembles that of a tooth fairy! The final Commander Keen [1], as the ultimate installation in this game series, contains believable enemies that fit the setting of a spaceship adventure. I particularly refer to the robotic enemies on board the Omegamatic [2], with regards to the Little Ampton, Robo Red, Shikadi Mine, Sparky, Sphereful, and Spirogrip (below). Although all images henceforth are taken from the Shikadi.net (2017) Commander Keen wiki, I will not reference these images academically since they originate from wiki stubs.
Figure 1: Characters and enemies in Commander Keen 5
Figure 2: Fatal obstacles in Commander Keen 5
2. Planes of vulnerability
Most platformer games are two-dimensional, running on an X- and Y- axis. It does not mean that the graphics are flat since a Z axis is introduced in the Commander Keen series [3]. However, since the player does not access the third-dimension during gameplay, the Commander Keen series remains a 2D platformer. The player moves the protagonist, Billy Blaze ("Commander Keen"), on the screen horizontally, vertically, and diagonally. Likewise, certain enemies move on these planes as well. My interest in the game- and level- design of Commander Keen 5 is the player's movement on these planes. Cognitively, the player must navigate fatal enemies and obstacles on these three planes and move the avatar accordingly.
Before addressing the planes of vulnerability, the movement of each enemy must be assessed. The Korath will be excluded from this description since it only appears in a secret level and is not fatal to Keen [4] in any way. Little Ampton is not fatal on the horizontal plane, but it will kill Keen if he is on a pole (vertical plane). The Little Ampton also pushes the player on the horizontal plane. Touching Robo Red is fatal, and he shoots out bullets diagonally. Since Robo Red moves on the horizontal plane and the range of his bullets are limited I will describe his threat on the horizon plane. Like Little Ampton, Shelly will push the player on the horizontal plane. Keen will be killed if the player jumps onto Shelley, and Shelley will fatally dive on Keen if he stands underneath Shelley. The Shikadi Mine follows Keen by floating through the air, exploding when near the player.
The Shikadi are fatal horizontally, sending deadly energy rays up poles. The Shikadi Master can teleport between different areas of the level (it appears in the closing stage only), shooting out energy balls that will kill Keen. The Shockshund is also fatal, jumping high (moving horizontally and vertically at the same time), and "barks" out lethal energy bits. The Slicestar moves either horizontally or vertically on a set trajectory, although a few move on a random diagonal trajectory. Sparky moves on the horizontal plane, occasionally charging at Keen. Like the Shikadi Mine, the Sphereful floats in the air, following keen. Spindred moves up and down on the vertical plane. Spirogrip moves quickly on the horizontal- and vertical- plane, killing Keen if he is in the way. Volte-face moves on a set (usually square) trajectory.
In conclusion: Keen can touch Little Ampton and Shelley without dying, but they can kill Keen on the vertical plane. Little Ampton, Shelley, Shikadi Mine, Shikadi, Shockshund, and Sparky can be destroyed, whilst Robo Red, Shikadi Master, Slicestar [5], Sphereful, Spindred, and Spirogrip are invulnerable. Shooting Volte-face will only stun it for a few seconds.
3. Levels and game areas
Figure 3: In-game illustration of the Omegamatic 
Figure 4: The Omegamatic as the world map
The player enters the Omegamatic from the bottom of the map (figure 4) [6]. Moving upwards, the player passes through the levels of Ion Ventilation System, and the Security System. After going up an elevator, the player has the option of accessing four levels (the Energy Flow System, Regulation Control Center [sic], Neutrino Burst Injector, and Brownian Motion Inducer) that each protects a machine that Keen must destroy. Each of these levels is guarded by a preceding level, a Defence Tunnel- (named Vlook, Burrh, Sorra, and Teln). After finishing these levels, the player can access a second elevator that will take the player up to the closing levels: the Gravitational Damping Hub, and the Quantum Explosion Dynamo. On the right-bottom of the map is a secret level outside the Omegamatic. I will not discuss this secret level since there are no obstacles related to the discussions in this essay.
4.1. Navigating the planes of vulnerability
As the player moves the avatar on the screen they must plan a strategy for overcoming enemies or obstacles. My analysis will not focus on problem-solving since it is outside the scope of this study [7]. Instead, I want to address the design of enemies and obstacles with regards to the planes of vulnerability.
An unaltered screenshot of each level will be presented followed by my analysis. I will not highlight every enemy- and obstacle- encounter, but focus on the level design of lethal encounters with relation to horizontal-, vertical-, diagonal-, and free-floating- planes. Dangerous areas will be highlighted with red, whilst safe planes of movement will be marked in green. Diagonal- and free- movement planes will be indicated with a circle. Additional notes are indicated in orange. The screenshots on the Shikadi.net website are taken in normal mode; my analysis will focus on hard mode. Subsequently, some enemies will only be described in my analysis and will not appear in the screenshots. Analysis will be marked numerically, following a typical gameplay trajectory from start to end.
4.2. Ion Ventilation System
Figure 5 and 6: Map and analysis of the Ion Ventilation System
In figure 6 (above) the player is immediately introduced to a horizontal plane of vulnerability (point 1). At point 2, the player is introduced to the vertical plane of vulnerability with Little Ampton sliding up and down the pole. At point 3, the player is introduced to the laser cannon that shoots downwards sporadically. Finally, Sphereful floats at point 4, introducing the player to free-floating enemy encounters.
4.3. Security Center
Figure 7 and 8: Map and analysis of the Ion Ventilation System
The Security Center [sic] introduces the player to more advanced planes of vulnerability. At point 1 the player is immediately faced with lethal planes that overlap: horizontal- and vertical- movements. Two Slicestars move up and down, Little Ampton slides up and down a pole and Sparky moves left to right and back. The player must decide how to overcome these obstacles.
At point 2 the player is introduced to the diagonal movement of Sparky. Although the player can avoid Sparky by jumping on the pole, Little Ampton will soon slide on the pole. Next to the pole is a laser cannon, doubling the vertical plane of vulnerability. At point 3 Sparky is in blocking horizontal- and vertical- movement of the player. The player must confront Sparky vertically without touching the robot, a recurring design concept in Commander Keen 5.
4.4. Defense Tunnel Vlook
Figure 9: Map of Defence Tunnel Vlook
Figure 10: Analysis of Defence Tunnel Vlook
At point 1 there is a Spindred bumping up and down whilst Sparkies patrol the area below. The player must time their descent before plunging down. At point 2 the player can take two paths towards level progressing; the lower path will force an encounter with Robo Red and Little Ampton sliding on the poles. The player must face a vertical encounter with Sparky at point 3, but now with the threat of Little Ampton sliding up and down the pole. At point 4 three Spindreds bounce whilst a Sparky patrols the horizontal plane. Point 5 is the most interesting area in this level: multiple fire helices on the floor with laser cannons shooting down. To traverse this area, the player must activate a free-moving platform. The jagged movements of the platform along with the laser cannons make this area dangerous; the player must calculate laser cannons' activation time whilst remaining on the moving platform.
4.5. Defense Tunnel Burrh
Figure 11: Map of Defence Tunnel Burrh
Figure 12: Analysis of Defence Tunnel Burrh
The player faces a double verticle lethal plane at the beginning of the game: Splicestar with Little Ampton on a stick (point 1). Point 2 features one horizontal- and three vertical- vulnerability planes. Whilst Robo Red patrols the room and Little Amptons slide up and down, a third menace lurks in this room: the Shikadi Mine. Whilst Splicestars move horizontally, the player must time their descent at point 3.
4.6. Defense Tunnel Sorra
Figure 13: Map of Defence Tunnel Sorra
Figure 14: Analysis of Defence Tunnel Sorra
Tunnel Sorra opens with five Splicestars: one moving horizontally, three vertically, and one Splicestar bumping diagonally (point 1). Point 2 is not particularly difficult to traverse, but the shape that the vulnerability planes form is interesting: A square, two vertical planes, and a horizontal plane. The player must momentarily slip pass a Volt-face at point 3 to get a key, whilst point 4 is a dangerous place: Robo Red, Little Ampton, and Sparky patrols the area.
4.7. Defense Tunnel Teln
