After the fierce battle in Level 20, the Marine enters the right side of the base through the water passage, then he goes down from the water and take a Defibrillator, which then creates a clone of the Marine who will come out as his ally. After the clone will appear, the two Marines enter the base. The Marine is forced to clear the base and make it to the opposite end of the level with the clone still alive.
In general, this level wouldn't be too hard for as long as you wouldn't rush it.
The least common enemy in the level, and are seen carrying light weapons such as the Assault Rifle CS-RC and the Lite Railgun v01 CS-HShot, while some of them are seen carrying the Grenade Plasmagun CS-Bloom. There are 9 Lites. They have 350 on all but impossible difficulty, where they have 130 HP.
The most common enemy in the level, there are 11 of them in this level, and are seen carrying heavy weapons such as the Heavy Railgun/Sniper Rifle, or the Rocket Launcher and are also seen carrying other weapons such as the Grenade Launcher. They have 500 HP on easy, normal, hard difficulties, but 210 HP on impossible difficulty.
- There is an unused hint near the end of the map, which is supposed to indicate that one of the playable characters has reached the end without his teammate. It reads "You can continue only with ally". However, it can only be seen if the player glitches through the final door.
- Eric Gurt said that Civil Security forces made a visual copy of Hero's suit and put regular soldier into it. It was an attempt to trick Marine with fake teammates.
- The clone has 600 HP no matter what battlesuit, but on impossible difficulty the lite clone has 130 HP, while the heavy clone has 150 HP.