Siege mode activates for a colony when Zombies do not have a valid path from the outside to the Banner. I.e. when the colony is fully enclosed within walls or a mound that zombies cannot cross. To avoid siege mode there has to be at least one entrance through which zombies can walk. The player should place Guards to protect that entrance.
For a normal colony each Colonist requires one meal per day to survive. During siege mode food consumption increases dramatically which can bring the colony to starvation very fast.
Mazes[]
Players sometimes build mazes in front of the entrance to increase the walking time for zombies and keep them within range for guards for a longer time. With very long mazes it can sometimes happen that siege mode does activate on its own and / or randomly. This means that the walking path trough the maze is too long for the game's AI and it is recommended to reduce the total path length.
Food Use[]
Food consumption in siege mode is about 10x that of regular food consumption. Food is automatically consumed during siege mode, without colonists having to visit the grocer or banner. This is because during siege mode, most colonists will likely be asleep and therefore unable to access the grocer/banner. During siege mode, each colonist will consume 0.4 meals per hour, directly from the stockpile. In comparison, the normal food consumption for a colonist is about 0.0417 meals per hour (totaling one meal per day, which the colonist will pick up at the grocer/banner). Food consumed during siege mode does not contribute to colony points.