Siege is a new mission type introduced in Lifeline. The game features a hidden timer counting down. When the timer reaches zero, a large number of zombies, accompanied by Juggernauts and Ferals, will attack Black Friday, attempting to breach the gates and kill the survivors inside. The player, along with AI-controlled soldiers, has to kill all attacking zombies to end the siege.
Once the siege ends, any rescued civilians or High Value Assets at the base will be airlifted out. Certain type of missions (e.g., Zed Hunts, Gather supplies, Investigate Artillery Site), if not completed before the siege ends, will disappear or marked as failed.The timer will reset and count down to the next siege.
At all times, a number in a triangle appears at the bottom of the minimap to let players know how close the upcoming siege is. Threat level will increase from 1 to 2, then to 3. When a siege begins, the number is replaced by a skull icon.
The Threat level consists of a 32 minute countdown. At 6 minutes it increases to level 2 and at 3 minutes to level 3.
Players can briefly delay sieges by setting the Tactics option at Ops Center to "Thin the Herd".
Thin the Herd has 5 outcomes which add time to the Threat level countdown timer. (Activates every 7 minutes)
- 25% chance to give a Zed Hunt mission.
- 16.7% chance for an "Excellent" success. Adds 4 minutes.
- 33.3% chance for a "Headway" success. Adds 2 minutes.
- 50% chance for a "Marginal" success. Adds 1 minute.
Completing the Zed Hunt adds 4 minutes.
- Players should begin to head back to their base when the threat level raises to 2.
- The best defensive position in the base is the roof of the building which has been set up as an improvised watchtower. This place has a line of sight to all entrances, as well as a supply locker conveniently placed nearby, allowing the player to quickly pick up ammunition or a heavier weapon if needed.
- When the player is standing on the ground, an assault rifle or LMG is good for engaging common infected or lone Freaks. Powerful .50 Caliber sniper rifles such as the M107 CQ are effective against hordes and Freaks, and are most effective when the player is standing on the ground (to make most of their penetrating rounds). Explosives and Grenade launchers are best used from a high vantage point so that the player can shoot through the obstacles like barricades and fences.
- Encumbered soldiers is less of a problem during a siege - it may be beneficial to ignore the weight limit and, for instance, equip a melee weapon, grenade launcher and an assault rifle or LMG (as well as ammo and meds) at the same time, to allow for tactical flexibility.
- If a civilian refugee is swarmed by a large horde of zombies, or attacked by a Feral or Juggernaut, do not hesitate to blast the zombies away with explosives- its not very realistic, but the survivor usually survives the explosion. Some trust will be lost, but it's better than losing a soldier or refugee.
- Set traps using Perimeter Defenses, and restock between sieges.
- Place mines outside the gates, especially locations likely to be passed by zombies, including approaches to the LZ. This allows for another line of defense to protect civilian refugees, though this may result in allies caught in the blast.
- The large fuel tank north of the base near the freeway entrance can be shot to obliterate nearby zeds (it will regenerate immediately after the siege).
- Arming AI-controlled soldiers with grenade launchers before sieges can make a huge difference. Players are also advised to keep a grenade launcher or other explosive handy to engage large groups of zombies or dangerous freaks.
- Cars can be parked to block entrances or funnel zombies in certain directions (Though it doesn't work well with arming AI-controlled characters with grenade launchers).
- The number of sieges a player survives will be noted on score sheet when the game ends.
- After reconnecting with Greyhound Two, if the player does not participate in the next Siege, the only soldier that survives is Royell Vincent.
- It is possible to bring cars into the base, by opening the Vehicle Ops gate, then quickly driving in through the open gate.