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. It is announced with various messages: "An Ambush! Curse them!", "Ambush!" etc.

As of 38a, they are exceptionally dangerous.

Ambushers

Goblins

Goblins will send a small force of attackers between the ranges of 4-8 soldiers^[Verify]. These forces are frequently led by one goblin with superior statistics and skill, and he is always of a different troop of the rest. If the leading goblin is killed, the rest of the squad will flee.

Usually they send more than one squad at once - up to 4 squads, each with his own leader. These squads may be from different goblin towers^[Verify].

Goblin are unable to navigate traps safely and can easily be stopped and defeated by simple cage or weapon traps.

Kobolds

Similar to Goblins, Kobolds will first send thieves dependent on your fort's population or 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^[Verify].

Humans

Hostile humans will not ambush.^[Verify]

Elves

Hostile elves will not ambush.^[Verify]

Tips

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 valuable, but shouldn't be the first line of defense, because the enemy crossbowmen 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, 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, 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 with a long reload time; 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 without any help. (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 not filled 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. 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.

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