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Source keepers (SKs), like invaders, are internally managed by the server and are designed to impede a player from harvesting in their room. Source Keeper rooms are the middle nine rooms in each sector. Some middle rooms (e.g. E65S35) do not have source keepers, but the surrounding 8 still do.
Source Keeper Basics
Source keepers have both ATTACK and RANGED_ATTACK body parts. They spawn from Source Keeper Lairs, move in range of the Source they are protecting and will attack any player that comes in range. They will not chase after creeps, they will stay by their source. There is a 300 tick delay between spawns if that SK has not been disturbed. Once an SK is damaged, the 300 tick countdown to a new spawn starts. If the old SK is not dead by the time the new SK spawns, the old one will suicide.