Several factors affect their power level:
- Progress in the Elder's quest: Affects their choice of gear, and matching stats with the settlers (which also scale with said progress).
- Success in previous raids: Based on how much damage was dealt, the next raid's raiders will have a damage multiplier, maximum 125% (if they keep failing), minimum 50% (if all settlers keep dying). It will only change in small increments and will reach the min/max after a few raids.
Raids will take place occasionally after dusk. They will not occur if the settlement has less than 3 settlers. Raiders will come from a specific direction, and they will spawn in and prepare for a short time before attacking. You can also see the raiders on the island map, so you will know where to defend.
Baseline Raid Frequency is between 1.5 and 2.5 hours. Each raid increases the next maximum cooldown by 30 minutes up to an upper limit of 5 hours.
Completing settlement boss quests reduces this maximum cooldown by 30 minutes again, never going below the base 2.5 hours.
Raiders will come in waves, each wave will consist of several small groups. The total number of raiders is determined by the amount of settlers in the settlement they are attacking. The minimum is 20 raiders, and there is no maximum (e.g. with 15 settlers there will be 53 raiders). The raiders will move towards the center of the settlement and the houses occupied by the settlers. Settlers usually stay in their rooms at night, but they will come out to fight the raiders as they approach their rooms. When a settler loses approximately half their health, they will retreat to their room to heal.
If there is no other settler to distract the raiders, they will go to the nearest occupied room, break in the door and attack the settler inside until either they or the settler are dead. If a settler is killed, the raiders will move on toward the center of the settlement attacking any settler they find on their way. The more settlers you have, the more they will try to kill before leaving. The exact numbers depend on difficulty, see the tables below:
Killing Raiders drops a random amount of coins and sometimes their weapon or part of their armor.
Raider attacks will continue until a message pops up that the raiders are leaving. They will flee to the edge of the zone and disappear, even ones fighting at the time.
If a settler dies during a raid, they do not respawn, but the same type has a very high chance to arrive soon after to be recruited.
Surrounding one's territory with a wall is useless. If they can't find a way, the raiders will break through the wall and enter!
Defending Against Raids
Among many techniques, traps can be used in defending against raids, such as a Kill Maze For Raiders. You can also take control of your settlers by clicking the Open settlement button (C by default), then Command, then selecting one or more settlers (via dragging a rectangle or left-clicking on one), then Follow me, and grouping up.
To make sure your settlers survive, equip them with armor and weapons.