The Outpost Base is a gigantic eight-legged moving fortress deployed by the Primers during the events of Earth Defense Force 5. It serves as the equivalent of the Quadruped Fortress seen in previous games, though mechanically it is closer to fighting Earth Eaters.
The Outpost Base is encountered in two states: in its "base mode" it is a static structure with the lower body and legs buried in the ground, while in its walking mode it moves slowly on eight enormous drill-tipped legs. In walking mode it stands 420 metres tall with a hull 520 metres across and a total width of 650 metres including the cannons.
In both cases it is armed with a huge number of cannons mounted all over its surface: these weapons have their own hitpoints and can be destroyed independently of the vehicle itself. They can only be destroyed if they are active, and cannot be locked onto with guided weapons in missions which do not require the destruction of the Base.
The vehicle has two hatches for deploying additional units: the first is a hatch on the rear superstructure, which the base uses to deploy Battle Drones. This hatch is indestructible and nothing can be done to interfere with its function short of destroying the Base.
The second hatch is mounted in the middle of the main body, and opens from time to time to deploy units, usually Colonists and Ants. This act reveals a glowing orange-red interior which serves as the vehicle's weak point, and is the only way to deal meaningful damage to the Base.
The Base has some interesting interactions with Air Raider strikes: in base mode it counts as terrain and piercing strikes like the 150mm Cannon and Spritefall will be stopped by its upper surface, but in walking mode it counts as an enemy and these strikes will pass right through it to hit weapons on the underside, though they cannot damage the weak point. More interestingly, bomber strikes and strafing runs pass through the vehicle at a height where they will damage the weak point if they hit it squarely while the underside hatch is open.
The Outpost Base is first encountered in Mission 22 (online 21) Scouting the Outpost, where it is stated to have appeared overnight and is presumed to have been dropped by one of the Primer Motherships. Five such structures are said to have been discovered around the world. The Outpost Bases have already been discovered to have a "perfect" air defence system and attempts to destroy them through airstrikes have failed. To this effect, an EDF ground force is deployed against one of them with orders to gather intel and if possible destroy the structure. However, this unit is quickly forced to retreat in the face of withering fire from the Base's gun batteries.
The EDF takes another shot at destroying the structure in Mission 30 (online 29) Destroy the Outpost: Plan A. More wary of the Base's firepower this time, a two-pronged strategy is employed: the "A" plan will be to approach the Outpost Base under the cover of a sandstorm and destroy it with demolition charges while underneath the effective range of its weapons, while a unit of B651 Titans stands by as a backup force.
While the EDF force succeeds in clearing a path to the Base, this only results in it revealing its true nature as a walking machine before the demolition team can plant their charges. Caught by surprise, the EDF fights a defensive action while unable to deal meaningful damage to the massive Primer weapon, with the Outpost Base ultimately leaving the combat area of its own volition.
The Outpost Base continues to interfere in other EDF operations (specifically trundling by in mission 35 (online 34) to annoy anyone who thought the mission looked easy) until it is decided to attempt a second assault with a coordinated land and air strike, with the ground team tasked with neutralizing the Base's anti-aircraft abilities. Despite a massive airstrike, the vehicle once again hauls itself into walking mode and engages the EDF forces: this time, however, the EDF does not give up, assaulting the hatch on the Base's underside until it collapses.
Another Outpost Base is encountered as the "boss" of DLC 1: in this case, the Base starts out in walking mode with all of its weapons active, and gains the ability to drop heavy armor Colonists.
The Outpost Base is incredibly heavily armed, with a variety of gun batteries mounted on its upper surface and even more on its underside.
The vehicle's primary weapons are three Fury Cannons, weapons similar in function to the Genocide Guns used by Ravager Motherships in previous games. When fired, they sweep a spread of white beams vertically in the direction the barrel is facing from the ground up into the sky, causing explosions wherever the beam strikes the ground. They are scaled down significantly in power compared to past appearances (with an explosion only dealing about 500 damage in singleplayer Inferno rather than the previous 10,000), though they deal damage twice, once for the beam and once for the explosion.
The next most powerful weapons form the No. 1 Battery. These nine giant laser cannons, six on the top and three on the underside, fire powerful beams straight forward: they are not aimed, but deal in the vicinity of 300,000 damage if they actually strike something. More dangerously under normal circumstances, each No. 1 Battery gun also has a pair of pop-out launchers, allowing each gun to fire volleys of six guided missiles.
The missiles are bright red with blue highlights and show up as enemies on the minimap: they are slow but relentless, have very strong tracking, and do not appear to have a limited flight time or limit to how many can be in flight at once. They are fairly weak with several impacts required to destroy larger buildings, but have a knockdown effect. They can be destroyed prior to impact with gunfire and locked onto by guided weapons, and even give Reload Credits like standard enemies, though they never drop items.
The No. 2 Battery consists of thirteen plasma cannons on the upper body that fire arcing green energy bolts. They are fairly weak, but have decent splash damage. They have a knockdown effect and can destroy buildings.
The No. 3 Battery contains the largest number of guns: it consists of fifty-two blue plasma cannons mounted on both the upper and lower surfaces, which fire blue bolts which have a knockdown effect and can destroy buildings. These guns can pivot for targeting and have a wide range of fire. This battery includes a series of guns mounted on a rotating disk atop the vehicle, which it only tends to use if a player character has got onto the upper surface.
The No. 4 Battery encompasses the remaining guns on top of the vehicle, and consists of two types of gun: the first is twenty red gun installations on the top of the base that fire red energy pulses, the second a ring of blue-coloured guns on the second disk atop the vehicle which function as anti-aircraft guns, firing the same red pulse projectiles.
The No. 5 Battery is similarly the remaining guns on the underside, and has two types of guns: fourteen blue laser installations that fire blue beams, and six green installations which fire a green laser in ten-second bursts, dealing continuous damage.
The Base will destroy any structure it touches with its legs, but outside of a scripted event where it destroys a couple of NPC Nixes it cannot actually deal damage with its footfalls.
Most of the Outpost Base's weaponry only has minimal damage scaling with increased difficulty, to the point that many of its weapons are not much more damaging on Inferno than they are on Easy.
- The Outpost Base has an associated city map which is not used in any mission that does not involve it. It is not particularly clear where in the world this map is supposed to actually be, as Japan is not noted for its sandstorms.
- While the base supposedly has perfect air defences, it does not in any way affect an Air Raider's ability to call in aircraft-based strikes or vehicles. Most likely this is due to frequent complaints about missions arbitrarily restricting the Air Raider's abilities in the two previous games.