Siege

Sieges are the current main game-over scenario for Towns. They can be toggled on or off via the options menu.

Description
Sieges are randomly occurring events that cause a widely variable number of assorted hostiles to spawn. Sieges on a "normal" setting range from two easy hostiles (i.e. spiders or evil badgers to over forty hostiles, including hobgoblins, direwolf alphas and more. Siege difficulty settings are: Disabled, Easy, Normal, Hard, and Insane, and can be adjusted by going to Settings-> Options-> Game-> Sieges (difficulty). The order of stages of the event are as follows: Sieges can happen in quick succession, and do not require the previous siege to be resolved before the next one begins. Players can toggle the auto-pause feature by accessing Settings-> Options-> Game-> Pause the game when a siege starts (ON/OFF)
 * The chance of a siege to appear is based on several factors, one of them is the number of soldiers in the town.
 * A siege can have a robbery.
 * Sieges can spawn on one of the edges of the surface map that has direct access to your town. Sieges do not spawn across rivers unless there is a bridge. The game, by default, is automatically paused and a message appears under the announcement heading informing the player that a siege has occurred. Clicking on that text takes the player to the site of the siege.
 * Sieges have a chance to come from underground, but only from the edges (like cave dungeon entrances).
 * When the game is unpaused, the hostiles will roam around the tile they spawned on for a short amount of time in order to space out a little.
 * The entire siege force will mill about momentarily, before engaging Townsfolk and Heroes in battle. They can go up and down ladders and can cross bridges.
 * Siege enemies choose random citizens when target.

Causes
It is currently unknown what exactly causes sieges. Some general thoughts are as follows:
 * Sieges do not seem to occur until the current Townsfolk population exceeds 7. In other words, if you have 3 of your original civilians die, and another 3 immigrants spawn, you should still not activate sieges.[v14b : False, had a siege with 4 townsfolk (month 2)]
 * Some say the frequency and intensity of sieges is attached to how many Townsfolk you have, and/or how low you have mined. It is hard to prove this as there is without a doubt some level of randomness involved - someone who has mined to the bottom floor could still spawn a siege of 3 Spiders. Sieges can occur even in towns where no mining has been done.

Surviving a siege
There are many ways to prepare yourself for a siege.
 * Equip all your Townsfolk with the best armor possible. Having at least bone armor if not iron armor is a good start before you start digging too deep.
 * Sieges, like immigrants, can only spawn on the very edge of the surface. Therefore, if you build a wall all the way around the map, no sieges can spawn (as of 0.40.2). Although, this also means no immigrants can spawn.
 * Building a bridge to the other side of the river can help. If the siege happens to spawn on the other side of the river, waiting until all monsters are on the bridge and then destroying the piece nearest to your town can strand the siege on the bridge. Then, destroying the pieces of the bridge the hostiles are on can drown them all.
 * Combining points 2 and 3 can be a very effective way of surviving - build a wall all the way around your side of the map, and have bridges across the water. This way, creatures are forced to spawn on the opposing side of the bridge - allowing you to use method 3 to destroy them.
 * Another effective method (although it uses admin commands so it can be considered cheating), is to put lava or water around the enemies and then unpause the game.The enemies must be trapped by the fluid in order for them to die otherwise they will only be pushed around. The water or lava will then kill the entire siege with very little risk to your townsfolk. Make sure you remove the water or lava after the siege has been defeated.
 * A great way to survive a siege is to set up traps around the entrances of your base so that its can weaken the monsters by health making it easier for your heroes or townies to finish them off.

The future for sieges
The devs have noted that sieges, in their current form, are unbalanced. Therefore, it can be assumed the entire system may be tweaked, or may even be a placeholder for a later system.