40d:Ambush

An ambush is a sudden event in fortress mode when a small force of enemy humanoids attempts to attack your fortress. While smaller in scope than a full siege, ambushes are not related to the number of dwarves in your fortress, and so can be triggered by relatively small populations. An ambush is not announced immediately, but instead is announced when your fortress becomes aware of the attackers, such as when they set off a trap or come close to your dwarves or pets. Not all traps trigger alert, specifically stone fall traps and weapon traps do not(38c), cage traps do(38c). You can still detect invaders from checking your traps "visually" - if the trap killed an invader the corpse and gear will be there. An ambush is announced with the message: "An Ambush! Curse them!". Ambushs tend to arrive with caravans but can happen any time. They are not limited to one race - Kobolds and goblins can arrive shortly after each other although they will not join forces as such.

The same alert will also occur if a non-citizen creature has been using the ambush skill and stops, e.g. while hunting. In this case, only the creature being stalked has been ambushed, and the player should not be alarmed.(38c)

Ambushers

Goblins

Goblins will send a small force of attackers numbering between 4-8 soldiers. These forces are frequently accompanied by one goblin with somewhat better gear, and he is always of a different troop than the rest.

Usually they send more than one squad at once - up to 4 squads, each with its own "leader". These squad leaders may be from different goblin towers.

Goblin are unable to navigate traps safely and can easily be killed or caught by simple weapon traps or cage traps. They also can not pass forbidden doors.

Kobolds

Kobolds will test your fortress by sending thieves dependent on your fort's population and wealth. If they are detected they will attempt to flee from you. Kobold soldiers - usually archers, though swordsmen and spearmen are possible - will begin to arrive if the Kobold thieves successfully steal any items; the number of successive soldiers and thieves who arrive will depend on how many items were stolen previously. Kobold melee soldiers have copper weapons and little or no armour, making them somewhat easy to dispatch, but ranged attackers can be deadly.

Humans

Hostile humans will not ambush.

Elves

Hostile elves will not ambush.

Tips

The following was blind-copied from Siege and still needs plenty of work. Please modify!

Active Defense

The very nature of an ambush makes it difficult to actively defend against one. If an ambush is detected and heads towards your fortress, standard siege tactics and defenses apply. However, until an ambush is detected, there is oftentimes simply too much ground to cover without spreading your forces too thinly to do any good. If you want to actively defend friendly workers and caravans and whatnot from potential ambush, probably the best way to do so is to deploy your forces strictly to areas that you want to keep secure. Building small towers with only underground access in these areas and stationing Marksdwarves in them is proven to be a very effective way of doing so.

One of the largest advantages an ambushing enemy has is the fact that they go undetected until they encounter something. This means that active detection can be extremely helpful in minimizing the damage dealt by an ambush by actually letting you react to the attack. One such way of doing so is by placing dwarves or chained animals in the direction from which an ambush is expected, sacrificing them to bait the ambushers into attacking them and thereby revealing their position. A less-lethal way is to cage animals (any animal will do) in such areas, albeit with the disadvantage of needing a lot more cages to cover an area. Finally, cage traps will usually capture ambushers and alert you to their presence, but they cover even less area than an animal cage.

War dogs and Siege weapons are much less effective against an ambush than they are against a siege. Trying to cover large areas with war dogs is liable to do little more than get them killed and an ambush out in the wild will often be too far away for a siege engine to attack. When ambushers do get into range, insufficient detection measures may let them get close enough to scare off the civilian siege operators before they can fire a shot.

Passive Defense

Traps will all attack goblin ambushers, and cage traps work against any ambushers while alerting you as they do so. Thus traps make good defenses, especially when placed in places that you know the enemy will be forced to go through.

Creating chokepoints in which to place detectors like animal cages and cage traps can greatly increase their effectiveness by reducing the number of ways the attackers can simply bypass them. If an ambush manages to make it to the outer edge of your defenses, an animal cage placed in such a chokepoint and backed up by several rows of traps is often enough to break the enemy and send them into retreat.

A moat - especially combined with drawbridges - is just as effective against detected ambushers as against a siege. However, it can also help keep outdoor workers such as woodcutters safe by sealing off access to areas that hold valuable resources. For example, isolating a large patch of trees behind a moat so that it can only be accessed from your fortress will allow you to harvest that lumber with literally zero threat from melee-weapon ambushers, and a line of fortifications behind that moat will reduce the threat posed by enemy archers and crossbowmen. Note that water-filled moats in cold or temperate climates can freeze during winter, so in those situations, filling that moat with water may not be a good idea.

Civilians

Like a siege, your normal dwarves will still attempt to do their jobs during an ambush. Even worse is the fact that ambushes are all too often detected when the ambushers reveal themselves by slaughtering civilians working outdoors. Standard siege measures will reduce further casualties, but that is little comfort to the woodcutters, hunters, etc. that are already full of arrows and stab wounds. To minimize the casualties taken in the first place, active and passive measures like those described above can be used to detect and combat incoming ambushes, giving you time to get your civilians to safety.

The best way to keep ambushers from killing your workers, however, is to keep your workers from going outside in the first place. 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. Just as importantly, finding a way to gain "outdoor" resources without actually going there (such as getting wood from Tower-cap farms or getting water and fish from a tunnel connecting a safe area to a river) will further reduce the need to send civilians into harm's way.