“I am not a vegetarian because I love animals; I am a vegetarian because I hate plants.”
Unknown
“The nice thing about being a celebrity is that if you bore people they think it’s their fault.”
Henry Kissinger
NS is a very complicated mod, and its mapping requires a whole array of new design considerations.
NS is a game of Marines versus Aliens. Aliens respawn in through a hive, and marines respawn in through Infantry Portals (IPs). The end goal of the game is to kill the other team, and the structures that spawn them in. But each team captures resources and builds through their tech tree, unlocking powerful equipment for the marines and evolving to various lifeforms of all sizes as the aliens. Rounds last anywhere from ten minutes to multiple hours, averaging about thirty minutes.
For starters, lets look at the most critical locations. Marines always begin the game in a set location named Marine Start (MS). Aliens, on the other hand, spawn in at a random of one of the three hive locations. Each map therefore has four critical locations; MS and three hives locations. It is not that the one hive is the only one used all game; aliens gain very powerful abilities by capturing and building a second hive, usually about eight minutes into a round. Marines often try to capture and defend the hive locations before this can happen. The result of a match is often determined by the aliens' success at capturing a second hive. The mapping guidelines explain that the runtime from MS to each hive should be roughly 30 seconds, and so should the runtime from one hive to any other hive. The traditional layout formation for these critical locations is a diamond layout.
So, there are four critical locations. But then there are the resource nodes. NS is a First Person Shooter set within a Real-Time Strategy game, and resource nozzles spread throughout the map can be capped by either team to provide resources, which are used to upgrade abilities, weapons and lifeforms, and to build structures. There are usually roughly ten resource nozzles per map, with one in every hive room and Marine Start.
Hence, there are usually around six additional, semi-critical locations. These have to be balanced vis-a-vis both teams, regardless of which hive the aliens begin with.
Since killing Hives is such a critical part of the game, the developers decided to add in a secondary method for doing so, aside from simply shooting them. This new option is called Siegeing. The marines can build structures called Siege Cannons, which can shoot through walls but only hit alien structures. In fact, marines tend to siege hives about 75% of the time, rather than attacking it directly.
As such, the rooms and architecture within a certain radius around the hive become very important as well. These need to be balanced for the epic battles that will occur on their ground, and are known as Siege Locations.
Aside from the AvP series, no other FPS game requires as much gameplay thought per room as NS. This is simply because other FPS' are grounded; it is only the ground and its architecture that truly matters, while the walls and ceilings are simply for decoration (windows and mountable objects would still be considered grounded). In Natural Selection, the base Alien class is a low-HP, high damage fighter, which has the ability to walk on any surface. They are far more effective when using ambushing tactics over direct combat.
This means that every surface of every room has to be considered in terms of accessibility for this class. An indent in a ceiling or a wall suddenly becomes a hideout for a sneaky alien.
Then there's all the other aspects of NS level design that everyone argues over; the general balance of maps and of rooms. In one sense, long hallways with little cover are Marine-Biased, as their weapons are all ranged. On the other hand, high ceilings allow the higher-lifeform aliens to maneuvre with more ease. And no matter how empty a room may be, every corner provides a perfect ambush location. NS maps are constantly pitting different balancing mechanisms against each other.
With four critical locations and six semi-critical locations per map, along with all of their relative siege opportunities, and mixed with fully three dimensional architecture, Natural Selection challenges mappers greatly in their design. I hope this helps you understand the game for which I designed Ns_Nexus.