40d:Siege

A siege is a special 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 draw bridges 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.

Siegers

Goblins

Goblins will send kidnappers and ambushers once your fort's population or wealth reaches a certain amount, and will start sieging once your population reaches 80. 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 evil human mercenaries) 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.

If you deflect enough sieges, the ruler of the goblin nation may lead a squad. He's equipped with extra-good quality equipment.

Humans

Unknown if humans still siege this version, someone confirm.

Elves

Unknown if elves still siege this version, someone confirm.

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.

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.)

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.

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.

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.

The Dwarves Stay Inside standing order, accessible through the o command, will prevent dwarves from going outside, but only after they walk to the entrance (or even a few tiles beyond it). Since many of your major defenses will be inside the fortress, this is only somewhat useful.
You should cancel all outdoor jobs -- un-designate trees for cutting, and set all freshly-cut logs and other outdoor objects that your dwarves might collect as forbidden.
Dwarves will still attempt to haul items and corpses, including the freshly-dropped weapons and armour of dead siegers. To prevent your civilians from running into the battle zone to pick them up, you should designate the items in the battlefield as forbidden. This can be tedious however because you will need to re-designate the entire battle area every few seconds as more units die and drop items.
You can also use the standing orders menu to tell dwarves to ignore bodies. However dwarves may still attempt to place their own comrades' bodies into coffins despite this order.^[Verify]
Possibly your best bet is to draft the dwarves in question (or simply all civilians) into one huge citizen-squad and station them somewhere out of harm's way. However, if any of your citizens are set to carry weapons or armor, they may decide to go collect equipment from slain goblins/dwarves during the siege, which means they'll run out into the fray. Keep an eye on this, and turn off their weapons/armor accordingly.
Providing indoor pastimes (like sculpture garden, zoo, meeting hall, or a meeting area zone) will make dwarves spend their break time in the fortress rather than outside. This will prevent dwarves from hanging around outside, which will lessen the likelihood that they will get caught outside when siegers arrive. Hunters, woodcutters, haulers and other dwarves who have business outside will still be at risk.