Part of the allure of Minerva: Metastasis is the fact that it doesn't have any of the custom models or textures that are usually perceived as necessary for mods. Rather than relying on a catalog of custom assets to create a new feel, it uses superb design to create something original but also faithful to Half-Life 2. It's something unique that couldn't exist in another game engine.
First, let's analyze the island. In terms of visuals, it uses the HL2 coastline textures and props, but in a different way, resulting in a very fresh but familiar feel to the level. Anyone who's played Half-Life 2 will immediately identify it as a continuation of that universe.
The structure of the island is definitely the most interesting aspect: A tall energy beam drills into the center of the island and serves as the primary landmark and focal point. For the first half of the map, that energy beam is the player's goal, and it's visible from any part of the entire island. Think of it as a Citadel.
... Except Adam Foster doesn't have several chapters worth of maps to emphasize the importance of this energy beam. Instead, he uses leading lines, silhouettes, and the layout of the island itself as tools for this purpose.
The layout roughly resembles a wheel: the beam is the hub, and the various rockforms and structures are spokes. All of these elements are converging towards the beam, although it feels natural and acceptable. As the player visits all of these locations, it builds up anticipation: "I wonder what the heck is in the center of this island!"
Even on a smaller magnitude, the level details converge towards the beam: The cliff silhouette slopes upwards, the grading of the shore slopes upwards, the waves break, the beached boat points... It's like the entire universe is directing you toward that energy beam. Yet, none of it is artificial; it's all a natural extension out of setting the level on an island in the Half-Life 2 universe.
The underground section has different strengths: everything is clustered together and connected, just like a real Cold War bunker reinforced by an interstellar army. There's two entrances to each room, which funnels the player towards his goal as he enters one way and exits the other.
Carcinogenesis also follows the Half-Life 2 tradition of linearity without feeling linear. It achieves this by guiding the player through design, rather than simply using a list of mission objectives or a map/radar display.
The player essentially travels in a circle around the island. This use of a "loop" is good primarily because (A) the gameplay seems less like a straight line (B) it's less work for the map author, as he can reuse the same map areas (C) it's logical, as the player tries to find a way around the enemy defenses.
On a smaller scale, the level design works to guide the player along this path - but instead of a locked door, Carcinogenesis takes more of a natural approach.
The map begins with the player being dropped on a beach next to the Combine base. Soldiers immediately begin firing on the player. Although they won't actually inflict much damage, the gunfire still creates a sense of urgency; the player thinks "I have to get out of here NOW." A convenient rockform near the player provides cover as the player reads Minerva's text message instructions. All of these combined factors force the player to run to the right, yet without the use of a locked door. (Though there IS a locked door on the left, the soldiers are too much of a threat to even consider it.)
This first area also efficiently communicates a lot of story information to the player: it is a former human outpost, now commandeered and fortified by the Combine.
This setpiece isn't so much about guiding the player as creating interesting combat: the two Combine soldiers start on the wood platform. When they see the player, one soldier runs down the slope to intercept the player under the bridge. Thus, the battle is more dynamic and creates an interesting multi-floor gunfight.
Valve loves using powerups to breadcrumb the player along, and it's effective since the player usually doesn't notice. This area at the Combine base is a variation on the beach in the beginning. The player runs out of a cave and is suddenly under fire from a Combine soldier. Assuming the soldier doesn't immediately die from a magnum headshot, the gunfire will direct the player's attention to the battery powerup. Once the player gets the battery, his attention shifts to the alternate path to the docks, which is a much more interesting fighting environment than the alley.
I also really like Foster's approach to detail: He puts it where it counts, where it will enhance something - where detail NEEDS TO BE! He doesn't worry about trivial things like texture misalignments in the corner of a dark room.
No one cares if there's an obvious seam between the cliff and the grass. The player is looking the other way, towards the middle of the island. Other mappers might have painstakingly put some sort of grass to rock transition overlay.
No one cares if the see-through metal catwalk ends abruptly and unrealistically. The player is focusing on the giant energy beam. Other mappers might have put some additional metal trim brush detail on the edge, but it's really quite trivial.
I REALLY like these claw-like Combine metal feet. But these things are rather small, so you'd think they're ridiculously unimportant...
... Yet the feet are important for the overall balance and silhouette of the outpost structure. There's funky metal structures and a refraction shader on the top of the tower, while there are small metal feet at the bottom. It's a balance of composition and detail.
I also like the railings that wrap around the catwalk as it enters the tower. It's as if the metal is "consuming" the platform, which is concurrent with theme of the Combine as an all-consuming destructive force.
Consider the use of normal mapping on these cliffs. It really pops out at the player and adds a lot of depth to the rocks. I'll be as bold as to say that most of Half-Life 2's cliffs didn't look this good.
Light isn't something thrown about a level to make it visible: it can be used to communicate information to the player or to emphasize certain rooms and objects. Now, this warm vs. cold, yellow vs. teal color motif really anything new in Half-Life 2 maps, but it's still worth mentioning since Foster pulls it off exceptionally well. I always find balanced lighting in his rooms, and it always "feels" right. Compare the power and intensity of the Combine spotlights to the old human-built cagelights. It almost seems as if the lighting tells a story.
... And in my opinion, that's why Carcinogenesis is a great map. Each element of the map works together in harmony to create a really polished experience. It's elegant. It's art. Of course, that's just my interpretation of the level.
-- Robert "Campaignjunkie" Yang, 27 August 2006