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.

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 is not necessarily true to ambushes. Please modify!

Active Defense

Put your entire military on duty. With luck, most of them are not sleeping, eating, or drinking. If a squad leader is 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.) The building of small towers over the map with only underground acess for your marksdwarves is proven to be very effective against ambushes.

War dogs are useful, but shouldn't be the first line of defense, unless you have quite a lot (12+) because enemy crossbowmen, if present, will quickly take care of them. Assign them to your military dwarves, or cage/chain 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 battle ends.

Siege weapons, catapults and ballistae, are likely to be ineffective against ambushes as a sudden ambush will likely be close enough to scare away the civilian siege operators.

Passive Defense

If you have no trust in your military's power (usually a good idea in the first years), keep all the dwarves inside and pull the ambushers into corridors with traps. Stone-fall traps are cheap and easy, but are one-shot traps (and you better not try reloading them during an ambush) ; weapon traps require weapons, but reload themselves after a few seconds, until they eventually get stuck. A 10-square-long entry hall filled with weapon traps will break most goblin ambushes. (How boring!)

A moat may provide a decent defense when combined with a drawbridge to either keep the goblins from entering, or to drop them right into the water. Substitute magma for some far more lethal results. Even an empty 1-tile wide channel is a fast and effective way to stop ambushers or to guide them into areas you want. Note that water-filled moats will freeze in biomes with a temperate or cold climate, and so should have a protective wall behind them to stop ambushers simply walking over the moat during the winter.

Civilians

Your normal dwarves will still attempt to do their jobs during an ambush. The Options->Dwarves Stay Inside option will prevent them from going outside, but only after walking to the entrance. Since many of your major defenses will be inside the fortress, this is only somewhat useful. Use doors and bridges to ensure your civilians stay out of the way. 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. Therefore, your best bet is to draft the dwarves in question (or simply all civilians) into one 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 ambush, which means they'll run out into the fray. Keep an eye on this, and turn off their weapons/armor accordingly or forbid the goblin's equipment (both quite annoying).

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 typically stop idle dwarves from hanging around outside. Hunters, woodcutters, fisher, haulers and other dwarves who have business outside will still be at risk.