Underground Fortress
| “ | An underground strongpoint that can only be destroyed from explosives placed inside. Players can spawn and stockpile items here. | „ |
| — In-game description |
The Underground Fortress is a Bunker modification designed to fill the role of a Safe House. It cannot be destroyed from the outside, so players must set explosives at the very bottom of it to destroy it.
Construction
It is built on a Large Structure Foundation at the center of 3 by 3 concrete Bunker Network.
It must be fully researched in the Weapons and Structure 
Tech Tree in order to be built.
The Underground Fortress option must first be selected in the Large Structure Foundation's interface. That option is only available if the Bunker Network is connected to a Bunker Base (through trenches or bunkers) that unlocked the Advanced Bunkers 
upgrade (requires 40 "Command" Infrastructure Modifications).
Once the option is selected three specific Large Structure Parts must be produced, transported, and deposited with a Crane onto the Large Structure Foundation in the correct order. First a Construction Parts must be placed, doing so locks in the selected Large Structure type. Next a Structure Parts and finally a Underground Fortress Parts must be placed to finish the Underground Fortress's construction. Once assembly is done, the Large Structure Foundation structure inside the Bunker is deleted and replaced by the Underground Fortress.
Once finished, the Underground Fortress is visible on the world map with its own unique icon 
but only by friendly players.
The concrete requires 48 hours to dry (longer if there is a Snow Storm or Rain Storm) during which the structure cannot be used and is more vulnerable.
Installing that Large Structure very slightly decreases (0.99) the Structural Integrity of its Bunker Network.
Destruction
It is immune to friendly fire.
Its health cannot go under 33% unless it hit on its underground part, below the stairs.
When destroyed it leaves behind a simple Tier 3 Bunker husk.
Use
Its interface can be accessed by soldiers inside the bunker by pressing E.
Defensive Position
it has two ladders on top that allows access to the Bunker Network. Inside the bunker the fortress stairs give access to a shelter underneath the bunker.
Forward Base
It acts as a Forward Base. Players can resupply at its stockpile, update their Map Intelligence by accessing its interface, and respawn there if they have set their spawn there and its stockpile has Soldier Supplies.
Deployment Point
Players can deploy to it from the Home Region.
Tactics
Gallery
A Construction Parts being deposited on a Large Structure Foundation with a Crane
Underground Fortress showcase during update 61 devstream
Underground Fortress interior showcase during update 61 devstream
Concept art for the Underground Fortress
