Protecting your fortress from intruders is a complex task. There's a variety of threats to consider, and many ways to counter them.

General Guidelines

Minimize fortress entrances: Have a strong distinction between inside and outside. This usually corresponds to underground and surface, but not always. Each point of contact needs to be hardened against attack. Don't make more entrances than necessary. If there is a useless opening, wall it off.

Concentric circles: One wall may not be enough. With the existence of door-destroying and bow-wielding attackers, a double wall between the inside and the outside is essential to fend off the worst assaults. The choke points between the circles are where you build traps and doors. Station troops between the walls.

Assume the worst: Build up your defenses before the enemy shows up. Plan on being sieged by hordes of goblin archers, trolls, kobold master thieves, giant eagles, angry elephants, and a bronze colossus all at once. Hopefully, you will never have to face that kind of threat, but being ready for anything is the best bet.

Threats

Wild animals

The first, and easiest, threat you will have to deal with is the local wildlife. Animals are easily excluded by the humble door or hatch, even if it's not forbidden. Elephants are not nearly as aggressive as they were in the days of Boatmurdered. The main threat from animals are those that steal items, like raccoons or eat your food, like bears.

With cage traps and an animal trainer (or Dungeon master) animals can be useful. Dwarves love zoos, merchants will gladly take your caged animals, and they provide a food source in times of need.

Thieves & snatchers

Any creature with a career title of thief or master thief has a few nasty abilities. First, they are invisible until spotted by your dwarves or tame animals. When spotted, there's an alert message, either "Protect the hoard!" or "Protect the children!" depending on the type of thief. Second, they can open forbidden doors. Not just for themselves either, the door stays open until a military dwarf "secures" the door, allowing any random creature to walk in. This can be a nasty surprise for players who aren't expecting it. Third, they bypass your traps. Thankfully, they don't disarm them the way they neutralize doors. This trap avoidance isn't perfect, there's some element of luck involved, and kobold thieves seem to be a lot better at it than goblins. A thief caught in a cage trap will be revealed automatically, even if no dwarf is in sight of the trap. Last of all, they make dangerous prisoners. Unlike military captives, thieves will break free and attack if you attempt to transfer or pit them.

Ambushes

An ambush is a small number of enemies (less than ten) that are invisible until spotted, like thieves, but somewhat easier to detect. The alert message is "An ambush! Curse them!" They skulk around the outside of your fortress, looking for a target of opportunity. They will often attack caravans as they move to your depot. Ambushers have random weapons, and typically have a leader (with a career title of "guard") with a different weapon from the rest.

If you have woodcutters or hunters roaming the surface, they are likely to be the first to encounter the ambush. This can provide a kind of early alert system, but you'll lose a few of them.

Sieges

See Siege

A siege is a large number of attackers that are announced as soon as they appear on the map. The alert message is "A vile force of darkness has arrived!" While siegers are on the map, the word "SIEGE" appears in the top corners of the screen. Siegers are organized into a number of squads, each squad having a different weapon choice. A siege can be led by a leader figure, often a master warrior. Goblin siegers sometimes bring along creatures such as trolls or beak dogs.

Enemy archers

Attackers with bows or crossbows are much, much more threatening than those with melee weapons. Out-shooting them with your marksdwarves is risky, and charging them with melee fighters is even worse. Advanced techniques are needed to shield your dwarves from the deadly rain of arrows.

Building destroyers

Some creatures have BUILDINGDESTROYER creature tokens in the data files. This gives them the fearful capacity of tearing apart your doors and bridges (and floodgates?). Trolls have this ability, as do some megabeasts. This doesn't allow creatures to knock down constructed walls.

Flying animals

Currently, the only flying creatures are wild animals, like the giant eagle. Be aware.

Megabeasts

A megabeast appears alone, with an alert message that mentions the beast by name. These creature are quite powerful. Approach with caution.

Death from below

There are threats below the surface as well as above. If you breach a chasm, underground river or lake, wild animals and hostile humanoids will occasionally emerge to attack your dwarves. They rarely attack in numbers, and can usually be defeated by ordinary traps and doors.

Underground threats may be more common in future versions. Underground roads may allow other civilizations to siege you from below. ^{v0.28.181.39c}

Elements

Soldiers

The core of any defense plan is the soldiers. A trained, armed, and armored military is the only way to bring the fight to the enemy. Keeping them in position is the tricky part.

Roughing it

Always have your soldiers carry food. They will each need a backpack to carry it. This keeps your soldiers from wandering off to eat. You can also have them carry water in waterskins or flasks, but this isn't recommended for the long term, as it keeps your soldiers from drinking alcohol. For an around the clock guard, have them sleep on the ground while on duty. Hopefully the sounds of combat will wake them up before they get killed. Sleeping on the floor causes unhappy thoughts.

Daylight training room

Put a weapon rack on the surface near your entrance and make it a training room. Training dwarves will be in position if there's trouble. This also helps prevent cave adaption in your military. You can use an archery target this way, too.

Doors and hatches

The most obvious way to keep any enemy out is with a door. You can forbid doors to keep humanoid enemies out, and your dwarves in. Outer doors can be closed against animals, to keep beloved pets from wandering into enemy fire. A floor hatch is just a vertical door.

Walls

Constructing walls around your entrance is an essential part of fortress defense. Currently, no creature can knock down a wall. Not only does it keep enemies out, your archers can stand on top of the wall and fire down. Keep in mind that this makes them vulnerable to enemy fire. To protect against that, build fortifications.

Fortifications

Fortifications block movement, and allow some missiles to pass through. Projectiles have a chance of being blocked, based on the firer's skill and distance to the fortification. There's no miss chance if the firer is adjacent to the fortification. Keep your marksdwarves close and keep enemies away. Build fortified firing platforms above ground level and put a nice wide moat between the wall and the enemy.

Moats and bridges

A retractable bridge over a moat is an almost airtight defense. The moat keeps building-destroyers away from the bridge, and the raised bridge blocks arrow fire. There are two important things to remember: Always build the bridge to raise towards the inside. The lever has to be pulled by a civilian, not a soldier.

The moat doesn't have to be filled with water or magma. Arguably, a dry moat is a better defense. If you want to build an escape rout out of your moat, make sure is leads to the outside, and is barred by a door (at least).

A moat with a non-retractable bridge is still potentially useful: It keeps enemy archers away from your fortifications, and it channels enemies into a narrow area.

A drawbridge without a moat is just a big remote control door. This doesn't work with retracting bridges!

Remote control doors

If you link a lever to a door, it becomes impossible for your dwarves to open and close it normally. Pulling the lever is the only way to open it. This keeps your dwarves locked in as well as keeping enemies out. It's unknown if thieves can open a door once it's linked.

A floodgate can be used just like a door, with two differences: A floodgate can be placed next to another floodgate, unlike a door, which needs to be adjacent to a wall. A floodgate is closed by default, and can only be opened with a lever. Be careful not to trap your dwarves.

A hatch cover can also be used this way.

As mentioned, a drawbridge works as a door

You can use automate a door by using a pressure plate instead of a lever, but there are many complications there.

Traps

The most reliable way to stop intruders is lots of traps. A thief's trap avoidance is subject to chance. A line of traps can wipe out an ambushes entirely, and inflict a lot of damage on a siege.

Stone fall trap

This is the easiest trap to build, so you can easily build them in large numbers. Building lots of them is an easy way to earn experience for your mechanic, and add to your fort's defenses at the same time. Surround every intersection and stairway.

Cage trap

A very strong type of trap. Maybe even too strong. Currently, even a wooden or glass cage can hold indefinitely any creature, even trolls and megabeasts. Also, a cage trap never fails. A large creature can shrug off damage from a stone or weapon trap, but nothing can escape from a cage. Use cage traps as your outermost traps to catch the occasional wandering animal. A wounded elephant or unicorn in your front courtyard is not good at all.

Weapon trap

The gold standard of traps. This is the only simple trap that works repeatedly without reloading. They do get jammed, however. View the trap with the items in room t mode, and if there's a corpse inside the trap, it's jammed. None of the weapons on a jammed trap will function. It may be wiser to have several weapon traps with fewer weapons, rather than a smaller number of ten-weapon traps.

Using crossbows in weapon traps avoids the problem of jamming, but they must be kept loaded with ammo.

Guard animals

Chained animals are sentries, not fighters. Most animals aren't strong enough to take more than one goblin warrior. Enemies with bows are even worse. The real purpose of guard animals is to spot thieves. Anything will do here, a cat is fine too. Don't use something useful, like a war dog. Put animals in narrow corridors, in places where enemy archers can't see them.

Meeting hall as defense

You can use a meeting hall zone to attract animals to a given area. This makes a pretty poor defense in general, but in the very early game, it's a way to defend your wagon and stockpiles from thieving animals. Remove the zone later, or it attracts idle dwarves and children.

Bait animals

Most enemies will go after your animals just as blindly as they attack your dwarves. An expendable chained animal can bait enemies into dangerous passages, even into places unconnected to your fortress.

Siege engines

Siege engines are not very useful in the current version. Catapults are only useful for training and stone disposal. Ballistas are deadly, but fire expensive ammunition, and hit both friend and foe alike.

Remember that siege operators are civilians. Fortunately, siege engines can fire through fortifications, just like normal projectiles.

Pillboxes and turrets

Build a tower specifically to post archers on. This lets you open fire before the enemy approaches your gates. A pillbox can be attached to your walls, or separate, so that the only access is from tunnels below. Carve fortifications on the second or third floor, so your dwarves can fire out. For extra usefulness, build a barracks, archery target, food stockpile, and dining room in or near the tower.

Siege engine turrets

If it's big enough, build a siege engine inside a pillbox. The device needs to be on ground level. Only a single tile of fortifications is needed to fire through the wall. You may want to build a moat or secondary wall to keep enemies at a distance. Position the tower to fire where invaders tend to congregate. Since siege operators are civilians, the "dwarves stay underground" order must be off.

Considerations

Civilians stay underground

This setting, in the orders and options menu is the easiest way to keep your non-military dwarves out of sight of the enemy. It is far from perfect, as dwarves will do the "entrance dance." They will attempt to leave the fortress, and only cancel jobs once they reach the surface.

It takes a truly airtight fortress to turn this setting off while there are still enemies outside. It's only safe to turn this setting off once the drawbridges and such have sealed off your fortress entirely. If there's even one exit, your dwarves will use it. Try testing this while it's safe: Raise the bridges, just like you would in a siege, and designate some trees for cutting. If there's a way out, your woodcutters will find it.

Screen the entrance. Build a simple wall around your entryway. This will keep your dwarves safe from enemy fire while doing the entrance dance.

Seal the entrance. Prevents the entry dance, but also blocks your soldiers, which can trap them underground.

Forbid dropped equipment and corpses. Mark every item on the battlefield as forbidden. This includes any items dropped by dead merchants or scuttled wagons.

Delete stockpiles and turn off tombs. As a preemptive measure, you can delete your stockpiles. Dwarves don't haul things if there's no stockpile to place them in. Turning off or removing coffins stops burials as well.

Keep them busy. Make a bunch of busy-work for your dwarves, just to keep them underground. It's not perfect but it helps. Time to re-organize your stockpiles.

Lever room

Be careful where you place the levers controlling your drawbridge. Or any lever at all, for that matter. Make sure that the entire path to each lever is underground or your dwarves might keep canceling the "pull lever" job. You can test this during peacetime, too. Try putting all your defense-related levers in a single room, and put a door on the entrance. Then you can lock your lever-puller inside to ensure rapid response time.

Another solution to the problem of rapid response time is to make your lever room double as a pump room. Pumping is a good way to build up your dwarves' attributes regardless of whether the pump is doing work or not. If you want a dedicated lever operator or three, turn off all their labors except pumping, and set the pumps up so that they can be operated exclusively by your dedicated lever operators. Rotate these positions every so often so the attribute gain will be distributed among multiple dwarves.

For the truly ambitious, the lever room could be spread over multiple levels, and the pumps could work together to power one or more artificial waterfalls. (Waterfalls work well in this case because their operation is not fortress-critical, and your dwarves like the mist they produce.)

Trade depot

Factoring in access to the trade depot adds a layer of complexity. Letting merchants in while keeping enemies out requires a careful balance. The merchants do reveal ambushes, and they can arrive in the middle of a siege.

Wagons need a three tile wide path to the depot. You will probably want to build the depot underground, so civilian dwarves can access the depot. Wagons can't use stairs, so you need a three-tile ramp, unless you can dig into the face of a cliff.

Airlock defenses/buffer zone

Build two walls, each with a drawbridge. Build the trade depot in the buffer zone between them. Keep the outer bridge open, and the inner one closed. When the merchants appear, put crossbows on the walls to guard their approach. Once all the merchants are safely inside, close the outer bridge. Once there's no enemies left in the buffer zone, open the inner bridge so your civilians can start loading up the depot.

The airlock pattern can be useful even without putting the depot there. Let a few siegers in at a time, and crush them. Reset the traps, Rest up the soldiers, and repeat.

Civilians trapped outdoors

Anything that blocks intruders will also block your dwarves. This can cause the problem of dwarves being trapped outside with the enemy. Having more than one entrance can be useful here. If you make these entrances accessible by drawbridge only, and keep the drawbridge up most of the time, having lots of entrances shouldn't be too much of a problem.

Branching corridors

Enemies will take the most direct path to your fortress. You can use this to your advantage. Have two paths to the fortress: a long, twisting, three-wide road, and a shorter, one tile wide, trap-filled passage. Attackers will usually prefer the short and deadly path. This makes a good line of fire for a ballista, too.

This isn't a perfectly reliable method, but surprisingly effective.

AI abuse

Taking advantage of pathfinding AI is a whole guide in itself. Try leaving a door un-forbidden during an attack. When the bad guys approach the door, forbid it, and the enemy will wander off. Unlock it again, and they turn around and head back towards the door again. You can get enemies to march back and forth over a set of traps this way. This might count as an exploit.

Roach motel

Build a long, narrow, and twisty passage, accessible from the outside, but unconnected to your fortress. Build as many traps as you like. Place a bait animal inside. Enemy attackers walk right in, and get torn apart by the traps. If any manage to make it to the end, and kill the useless animal, they're surrounded by traps, and no closer to your fortress.

If the roach motel is deep enough underground, you can build a tunnel above it, channel down, and mark the channel a pit/pond. That way, you can "reload" a new bait animal.

Water sources

Access to water can be important. Wounded dwarves need water, so if there's not an underground water source, you'll lose valuable soldiers to thirst. Try to have a well or cistern your dwarves can use safely. Remember to keep an extra bucket or two available.

Examples

3 Bridges

An example of some advanced defensive construction tactics to deal with vile forces of "any" size. (See picture).

Bridge 1 seals off the entire base
Bridge 2 forces everyone to take the long, winding, heavily trapped path of death.
Bridge 3 seals the inside of the fortress

Clever triggering of the bridges allows you to break the hostile forces into smaller chunks to be trapped in the courtyard while being caught in traps and a crossfire of arrows from the fortifications around.

40d:Defense guide