40d:Siege

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 the Invasion'.

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. Unlike a caravan passing your fort due to an inaccessible trade depot, the 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.

Siegers

Goblins

Goblins will send kidnappers and ambushers once your fort's population or wealth reaches a certain amount^[Verify], and will start sieging once your total population reaches 80. (Dead dwarves count)^[Verify]. Sieges will increase in intensity depending only on how many previous sieges you have survived - a population higher than 80 does not increase the number of goblin siegers.

They arrive in squads of about 15 goblins each, frequently led by individual goblin weapon masters (or even babysnatched/refugee humans and elves) and sometimes mounted on beak dogs, and occasionally accompanied by up to 3 squads of 5-8 trolls. They frequently are split into separate squads placed on different map edges. 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.

Trolls are the goblin "siege engines". They are faster than beak dogs, and will make for buildings and start demolishing. Locked doors will keep the goblins out, but can be demolished by trolls. Note that Constructions are treated as inert terrain objects and therefore can't be destroyed by trolls.

If you deflect enough sieges, the ruler of the goblin nation may lead a squad. He's equipped with extra-good 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 last goblin survivors retreat at full pelt; it is not always necessary to exterminate them.

Humans

Humans can siege your fortress, letting too many caravans get lost to enemies may provoke them. On entry to the map they will set up a Campfire and wait there for a while, making attacks of opportunity on dwarves that come to the surface, before taking the final headlong charge in much the same way as goblins.

The message is slightly different from the goblin sieges: "The enemy have come and are laying siege to the fortress".

They may be mounted on horses.

They may eventually send a diplomat, who will parley with your leader and offer a peace treaty.

Elves

As of Version 39f it is possible for Elves to lay siege to your fortress. You will get the standard message notifying you of a Siege; however, you will not be able to see any enemy units and the SIEGE banner across the top will instantly disappear. This is because all elven siegers enter the map sneaking; isn't that fun? The Elven squads typically consist of a mix of swordselves and wrestlers, all of whom will be using wooden equipment. Later sieges may also feature archers and spearelves, again with wooden weapons. The first time this happens, if your military consists of untrained dwarve, an Elven siege could be an extreme problem. Otherwise, most will not have much trouble resisting the sieges. Difficulty may eventually increase, but this has not been confirmed.

Elves may be mounted on unicorns.

Kobolds

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

Megabeasts are a siege consisting of one enemy. A certain wealth or a population of around 100 may trigger one.^[Verify] A megabeast, such as a bronze colossus, dragon, or titan, will arrive on the map and head towards your fortress.

Megabeasts rely entirely on [BUILDING_DESTROYER:2] to path to your fortress. Unlike other sieges, they can be stopped simply by shutting a door behind another door. Vanilla megabeasts are easy to defeat and by default only 20% will survive worldgen.^{v0.28.181.40d} Can be set higher via /data/init/worldgen.txt.

Increasing [DAMBLOCK] or [SIZE] can help buff up megabeasts, as well as editing their body to be more complex (realistic dragons with scales, for example) and setting them to be made out of certain materials (steel or adamantine, for example)^[Verify]

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 choke points, 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. They can also shoot from different Z levels, use this. (This is why you build fortifications.)
- A good tip for your military is to make sure that you take out goblin squads early. They tend to arrive all over the map, so you have a chance to strike while they are still scattered. Send groups of dwarves large enough you know you will win with no causualties (normally, a modestly trained group around three-quarters the size of a goblin group has nothing to fear unless there are crossbows, but use extra dwarves just in case) to each squad, so that you don't have to deal with 50 goblins beating down your door in one massive group. In such large numbers, goblins are far more likely to seriously injure or kill even the hardiest of dwarven warriors, while when they are in their initial squads dwarves have much less to fear.

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, catapults and ballistae, 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 up to three real-time minutes 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 some megabeasts can be siezed by these) and there are a lot of entertaining ways of dealing with captured creatures once their friends have been beaten back.

Locked (forbidden) doors will keep invaders out indefinitely, if locked before they seize the door. Doors won't keep siege trolls out, but drawbridges will. 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, a well 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. Even when not filled, a 1-tile wide channel is a fast and effective way of stopping besiegers or to guide them into areas you want. Note that water in moats may freeze over during winter (in some maps) providing a possible, if temporary, weak point.

Civilians

Your dwarves will still attempt to do their jobs during a siege, including cutting down trees or hauling in items and corpses from outdoors. 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. There are several strategies to preserve your civilians' lives, none of them perfect.