- (If looking for articles on catapults and ballistae, see Siege Engines.)
A siege is a special, fun time in Fortress mode when an army attempts to attack and kill all of your dwarves. It is at this time you should activate your military, keep civilians indoors, raise the drawbridges and pray you have your defenses ready.
During a siege, the option on the main menu 'Abandon Fortress' changes to 'Succumb to Siege'.
A siege is not to be confused with other types of hostile encounters - if you are besieged you will know. If you are unsure, you are not under siege. When you receive a siege, you receive a full-screen message "A vile force of darkness has arrived!" and the top of the screen reads "SIEGE" in yellow and red. Siegers are immediately visible at the map edge, whereas ambushers or thieves are not.
During a siege supply lines are cut, and no caravans will visit your fortress. During a siege, traders don't appear on the map at all, and no message informs you of this. However, caravans that are already on the map will continue to your fortress as normal, assuming the besiegers don't kill them.
Goblins arrive in squads of about 15, frequently led by individual goblin weapon masters (or even babysnatched humans) and sometimes mounted on beak dogs, and occasionally accompanied by up to 3 squads of 5-8 trolls. They are split into separate squads scattered along the left edge of the map. The first siege you see with a given fort might consist of as little as a single unmounted squad with no trolls, but the goblin forces will escalate in size as the game progresses. Later on you may be seeing 100 or more goblins show up in a single siege, all mounted, with 10 to 20 trolls.
If you deflect enough sieges, the ruler of the goblin nation may lead a squad. He's always equipped with high-quality equipment.
Goblins are less than stalwart, and once a siege sustains significant casualties, there will come a rousing cry of "Screw you guys, I'm going home" as the survivors retreat at full pelt.
Humans attack you if you make too many caravans vanish. Their large stature allows them to swing heavy mauls, and they can bypass any traps spied out by their diplomats. After the initial siege, there is the possibility that the humans will come to you in the form of a diplomat seeking peace. It is unknown what causes the Humans to seek peace, but it is assumed to be determined by their losses in combat.
Elves might attack you if you kill off their caravanVerify. Also, if their diplomat informs you of a tree cutting quota, and you break it, they will bring large numbers of bowmen (bowelves?) to turn your dwarves into pincushions. This is not strictly a siege as they will lie in ambush in the forest rather than enter your filthy meat-strewn smoking hole in the ground.
Similar to Goblins, Kobolds will first send thieves dependent on your fort's population or, rather, wealth. Kobold archers will begin to arrive if the Kobold thieves successfully steal any items - the number of successive archers and thieves who arrive will depend on how many items were stolen previously.
Kobold archers tend not to directly siege your fort, but prefer to pick off individual dwarves who may be working in the surrounding wilderness. They will leave once their arrows have been exhausted.
Megabeasts are a siege consisting of a single, enormously powerful enemy creature. A megabeast, such as a bronze colossus, dragon, or titan, will arrive on the map and tend to mill around near their entrance location, killing anything that happens to be nearby.
 Defending against a Siege
 Active Defense
- Put your entire military on duty. With luck, most of them are not sleeping, eating, or drinking. If a squad leader is doing anything of that sort, replace him with a more alert squad member (the squad always clusters about the leader. If the leader's eating, the squad will guard the table). Place melee units at major chokepoints, so they can meet the enemy head on, but try to keep them out of direct fire from enemy missile users. Place your own marksdwarves where they can rain death down on the enemies (this is why you build fortifications).
- War dogs are valuable, but shouldn't be the first line of defense, because the enemy bowmen will quickly take care of them. Assign them to your military dwarves, or cage them before the siege, and release them via lever/pressure plate as the enemy is rounding a blind corner. They're also useful for clearing the field once the siege ends.
- Siege weapons can be effective during a siege, but can also be entirely useless. They don't have a wide field of fire, so you'll need to design your fortress ahead of time to funnel your attackers into the weapons' field of fire and then delay them with winding passages while in range. To use them effectively, you really need trained Siege Operators for the task, since siege weapons take unacceptably long times for inexperienced operators to load, and the weapons cannot be fired at a precise time; they will fire whenever the operator shows up. Fire early and often: siege operators are civilians, and will run away once the oncoming hordes get too close.
 Passive Defense
- If you have no trust in your military's power, keep all the dwarves inside and draw the besiegers into corridors with traps. Stone-fall traps are cheap and easy, but work only once before needing to be reset; weapon traps require weapons (and ammunition, in the case of ranged weapon traps), but reload themselves after a few seconds, until their components eventually get stuck due to all the gore. A 10-square-long entry hall filled with weapon traps will break most goblin sieges without any help. Cage traps are one guaranteed removal in most cases (even megabeasts can be seized by these) and there are a lot of entertaining ways of dealing with captured creatures once their friends have been beaten back.
- Locked doors will keep invaders out indefinitely, if locked before they seize the door. Doors won't keep siege trolls out, but drawbridges willVerify. Closing all entrances will result in enemies milling around outside your walls without a destination. The siege will end after some (LONG) time have passed, but if you intend to sit it through, make sure you have enough wood and a food source.
- A moat can provide a decent defense when combined with a drawbridge to either keep the goblins from entering, or to drop them right into the water. Magma may be substituted for far more lethal results.
- Yet another strategy for dealing with sieges is building a magma moat around the entire outer border of the map at least 2-3 tiles wide - this will end all sieges instantly, but it will also end all dwarven and elven caravans just as quickly. Thankfully, the human and dwarven caravans always seem to show up at your road (and the elves just don't matter), so with the addition of the occasional bridge (and perhaps more channels to funnel anything spawning on the bridges through a trapped hallway just to be sure) you can have your instant-win sieges AND not infuriate your neighbors. You may wish to drain the magma from time to time to collect the staggering amounts of weapons and armor that get left behind. Also, this will greatly reduce the amount of wildlife you encounter, for good or ill.
- Your dwarves will still attempt to do their jobs during a siege, including cutting down trees or hauling in items and corpses from outdoors. Since many of your major defenses will be inside the fortress, this is only somewhat useful. Dwarves will run from invaders, but only after getting within crossbow-range, so their self-preservation skills are lackluster when the enemy has ranged weapons, or moves more quickly than them. Making a burrow inside your fortress and activating it through the millitary-menu will make them all run inside, saving them from certain death. (Create a burrow using W, then press M for the Millitary-menu, assign the dwarves to a burrow using Alerts (A))
- Providing indoor pastimes (like statue gardens, zoos, or meeting halls) will make dwarves spend their break time in the fortress rather than outside. This at least reduces the number of dwarves hanging out in front of the gates. Hunters, woodcutters, haulers and other dwarves who have business outside will still be at risk.
- Lock the front doors. If you still have valuable dwarves outside looking for a way in, you might try locking and unlocking doors just as dwarves arrive so as to keep as many dwarves in as possible.