40d:Fortress defense

There are two important things to consider when designing defenses for your fortress. First, you must protect the fortress itself. Second, protecting your dwarves is also often a priority. These two goals can often be rather divergent, as without careful planning your dwarves may wander the open countryside to collect herbs, cut trees, hunt, fish, or otherwise just enjoy nature, and while outside your fortress are vulnerable.

Entrance design and traps

You can help your soldiers a great deal by sensible entrance design and use of traps and siege engines. Simple approaches include clustering stone fall traps or cage traps in your entrance. More care is needed when placing ballistas or catapults as they can hurt your dwarves too. Below are some example designs of more elaborate possibilities.

Fortification based entrance designs

Keep in mind that the entrance to defensive chokepoints should be outside. If you try to set up your defense somewhat underground, ordering your dwarves to stay inside has the result that your carefully designed 'kill zone' will be chock full of your own dwarves at the critical moment.

This particular design works well with plenty of archers, siege engines, and other ranged weaponry.

═ - Wall
╬ - fortification
+ - Floor, the path the attackers must take
\ - pit (filled with spikes, water, magma, whatever you prefer)
@ - Marksdwarves
1, 2, 3 - Bridges (3 shown) over pits

|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|E|N|T|R|A|N|C|E|+
|═|═|╦|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|1|1|1|═|╦|═|═
|+|@|╬|\|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|\|╬|@|+
|+|@|╬|\|+|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\|2|2|2|\|╬|@|+
|+|@|╬|\|+|\|\|+|\|\|\|+|\|\|\|+|\|\|\|+|+|+|+|\|╬|@|+
|+|@|╬|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|+|+|\|╬|@|+
|+|@|╬|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|+|+|\|╬|@|+
|+|@|╬|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|\|+|+|+|\|╬|@|+
|+|@|╬|\|\|+|\|\|\|+|\|\|\|+|\|\|\|+|\|\|+|+|+|\|╬|@|+
|+|@|╬|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\|\|+|+|+|\|╬|@|+
|+|+|╚|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╬|╗|\|+|+|+|\|╬|@|+
|+|+|+|@|@|@|@|@|@|@|@|@|@|@|@|@|@|@|╬|\|+|+|+|\|╬|@|+
|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|@|╬|\|+|+|+|\|╬|@|+
|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|+|@|╬|\|+|+|+|\|╬|@|+
|═|═|═|═|═|═|═|═|═|═|═|═|═|═|╗|+|+|@|╬|\|+|+|+|\|╬|@|+
|█|█|█|█|█|█|█|█|█|█|█|█|█|█|║|+|+|@|╬|\|+|+|+|\|╬|@|+
|█|█|█|█|█|█|█|█|█|█|█|█|█|█|║|+|+|@|╬|\|+|+|+|\|╬|@|+
|█|█|█|█|█|█|█|█|█|█|█|█|█|█|║|+|+|@|╬|\|3|3|3|\|╬|@|+

The 3 tile wide lane is for traders, so if you have your trade depot before this location, cut it down to a 1 tile lane to slow down invaders more.

Retract bridge 2 to force invaders to take the long way, first running along the wall (to the west in this diagram) and then zig-zagging back for maximum exposure to your ranged units on the walls above them. The pits at the base of the walls are necessary to target down one z-level at enemy units.

(Note - Traders will NOT stop at your fortress unless there is a clear, 3-wide path to a Trade Depot when they arrive. Bridge 2 should be kept down if you want/expect Traders.)

Bridges 1 and 3 can be put down to allow enemies into this pit entrance, then retracted again to trap them in the kill zone. Note - Attackers must have a complete "path" to where they want to go, so bridge 3 must be down to lure them in.

The "Reverse Battlement" design

  |'|'|'|'|'|║|+|+|+|║|'|'|'|'|'
F |═|═|═|═|═|║|+|+|+|║|═|═|═|═|═
O |.|.|.|.|.|╬|@|+|+|║|.|g|.|g|.
R |.|.|.|g|.|╬|@|+|+|║|.|.|.|.|g
T |.|.|.|.|.|╬|@|+|+|║|.|.|g|.|.  <- Direction  <-
R |.|.|.|.|g|╬|@|+|+|║|g|.|.|g|g   <-  of attack <-
E |.|.|.|.|.|╬|@|+|+|║|.|g|.|.|.
S |═|═|═|═|═|║|+|+|+|║|═|═|═|═|═
S |'|'|'|'|'|║|+|+|+|║|'|'|'|'|'

. = ground level
+ = bridge/overpass
╬ = fortification
═ = wall, channel or other obstacle (to force path of attackers)
@ = marksdwarf
g = goblin attackers

Note that in this diagram, the fortress interior is to the West, and the enemy forces come from the East. The marksdwarves on the bridge with the fortifications are one level above the goblins (or other attackers), who will pass under the bridge and charge on toward the west. As the first clear from under the bridge, they are targeted from behind (which is one level above), as the marksdwarves wait in ambush. This allows the marksdwarves to face far fewer enemies at any one time, at least to begin with, and any enemy archers must clear the bridge, take their lumps, and then return fire back the other way before the marksdwarves are ever under attack.

For extra safety, hollow this tunnel out from under a ledge of the mountain (so it counts as "surface" and Dwarfs can "stay inside"). The bridge part can then be made out of construction, as soldiers can be ordered to go outside anyway.

If you're feeling especially nasty, make the tunnel really long into the mountain and add a ballista battery (see below). In my current version of the fortress, the goblins have to cross a long series of drawbridges to even get inside the mountain, so the ballista dwarf gets a lot of shots, and I can launch any escaping troops into the air.

(Adding ammo stockpiles, of your best quality bolts, to these stations will speed up reloading for longer sieges/battles. Even adding small, convenient food and alcohol stockpiles is not unheard of. Some designers place access to/from archery ranges very close to these stations, for faster deployment.)

Flooded entrance

Using a chamber as your entrance alongside a chamber full of water and some machinery you can flood or drain the entrance at will.

The basic premise requires two levers, two screw pumps and two gear assemblies. The amount of power required and the number of additional components needed to get the power to the screw pumps varies depending on distance/setup. One pump is placed to draw from chamber 1 and dump into chamber 2. The other is set in reverse. A gear assembly is placed next to each pump and connected to the main power system. Each gear is linked to a lever. Now at the flip of a switch you can submerge your entrance with water or magma for easy, secure defense against creatures that aren't amphibious or magma-dwelling, depending.

The picture above shows the design in action. The green pump is currently on while the red has been disconnected through the grey marked axle. The yellow X is just to mark that there is a channel under the axle.

Water trap

Level 0

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-|>|.|.|\|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|/|.|.|-|>
-|>|.|.|\|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|/|.|.|-|>
=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=|=

Level -1

=|=|=|=|=|=|X|X|=|=|=|=|=|=|=|=|=|=|=|=|X|X|=|=|=|=|=|=
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=|=|=|=|=|=|X|X|=|=|=|=|=|=|=|=|=|=|=|=|X|X|=|=|=|=|=|=

Level -2

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=|=|=|=|=|\|.|.|/|=|=|=|=|=|=|=|=|=|=|\|.|.|/|=|=|=|=|=
=|=|=|=|=|\|.|.|/|=|=|=|=|=|=|=|=|=|=|\|.|.|/|=|=|=|=|=
=|=|=|=|=|\|.|.|/|=|=|=|=|=|=|=|=|=|=|\|.|.|/|=|=|=|=|=
=|=|=|=|=|=|x|x|=|=|=|=|=|=|=|=|=|=|=|=|x|x|=|=|=|=|=|=

= - Wall
. - Floor
_ - open Space
\ - ramp (direction should be clear)
X - Inflow
x - Outflow

If enemies are in the middle of Level -1, open the inflow, then the water will first trap, and then drown them. If the pit is full, close the in- and open the outflow. You can automate this by using pressure plates, or if you want to have more fun, replace the water with magma (pressure plates and floodgates have to be magma-save then).

Siege Engines

One effective way to have Siege engines (help) defend your fortress is:

+ - floor
═ - wall
▼ - ramp
\ - channel
╬ - fortification

One ballista vs 3-wide hallway

|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|╦|═|═|═|═|═
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|║|▐|▀|▀|
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|╬|◄|═|«
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|║|▐|▄|▄|
|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|╩|═|═|═|═|═

Using this design you can cripple an army using a well timed volley. The hallway can be much longer than shown if you wish, as ballistae have extended ranges well over 100 tiles. The channeled area is necessary, as civilians (siege operators are "civilians") will run when enemies get within about 5-10 tiles of them, regardless of the actual path to that threat.

3 (or more!) ballistae can be put into a "battery" if overlapped - one per tile-width of the hallway, with each ballista aiming down their row of tiles.

                                                                |╦|═|═|═|
|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|╦|═|═|╦|═|═|║|▐|▀|▀|
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|║|    ║|▐|▀|▀|◄|═|«  (~ammo~)
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|╬|▐|▀|▀|◄|═|«|▐|▄|▄|
|E|n|t|r|a|n|c|e|+|+|+|+|+|+|+|+|+|+|+|+|▼|\|\|\|\|\|║|◄|═|«|▐|▄|▄| (~ammo~)
|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|═|╩|▐|▄|▄| (~ammo~)
                                                      |═|═|═|═|═|═|═|═|═|

Be sure to use fortifications to prevent dwarfs from wandering in front of the ballista to their deaths. If desired (and you have the ~~man~~dwarfpower to spare), catapults may be put behind those, as they shoot safely over workers in front of them. Altho' less effective than ballistae, it's a little more firepower - and that can't be a bad thing.

Dropping the Hammer

Replace the side wall of a part of your main entranceway with a drawbridge, big enough so it spans the whole hallway. To prevent enemies from wrecking it, you could dig a channel in front of it so its protected while its raised. Link the drawbridge up to a pressure plate or lever, whatever you prefer. Whenever you feel like it, activate the trap, and watch the drawbridge fall directly into the hall, utterly squashing anything beneath it. This can be done with minimal effort and used to smash invaders, unwanted immigrants, nobles, or simply to destroy your garbage. This is based on the 'Dwarven Atom Smasher' device mentioned elsewhere in the wiki.

Forest of Spears

Try planting a bunch of Upright Spear/Spikes in your main hallway, and link them all up to one lever. When invaders come near, give the order to pull the lever, but set it to Repeat. The lever will continuously be pulled and the spikes come up and down repeatedly, perforating most enemies in seconds. Note that this takes a lot of time and mechanisms to pull off.

Tar Baby

Enemies will hunt down and kill friendly tame animals wandering outside if they have nothing better to do. Put a puppy on a chain in some random spot outside, build a few columns around it to reduce the chance of them shooting it, and trap that area to hell and back.

Obstacle Course

Combined with some intentional cave-ins, lots of cage traps, some weapon and stone fall traps, combined with channels and pressure plates can be sure-fire way to fend off an invading goblin army.

Create a small area 1 tile wide prefferably, and channel it out leaving one support under it. This MUST be connected to your base by a bridge so enemies will take it. the place a pressure plate at the end, linked to the support and another bridge into your fortress. line this area with weapon traps and line the exits of the subterranean area underneath this with cage traps. the area underneath must also lead to your base so the enemies will walk into the cages. then build another area just like this except have the entrance bridge be retracted/raised and have the first pressure plate link to it, so it is an endless game of at and mouse (sort of)

I have a screenshot but I don't know how to upload

Defending your dwarves

The best way to prevent dwarves outside from being ambushed and slaughtered by hostile creatures is to keep them from going outside. Unfortunately, there are valuable commodities like wood which are hard to acquire inside.

The next best thing is to provide defenses which protect your dwarves while outside. On the truly labor intensive end, you can fully enclose areas of wilderness you wish to utilize in walls or behind moats with the only access being from within your base. While faster, the moat is less effective because it can be seen over, and unless the area beyond the moat also has no access to any entrance to the fortress (and no dwarves) you will still be vulnerable to archers. Hostile creatures, even 'invisible' ones like ambushers, start at map edges and travel across the map - they will only spawn in regions where they can path to a dwarf. By controlling which areas have access to paths to dwarves, you can force all hostile forces to appear in predictable and limited areas. Basically, creating artificially constrained outdoors areas for dwarves to work in is like keeping your dwarves inside - you're only vulnerable while establishing the defense system, afterward its part of the fortress for most purposes.

Another options for outside defenses is scattered traps. Most hostile forces will flee if they take enough casualties, and stone-fall traps can be quite damaging to goblins and are easy to set up. Cage traps work even on Bronze Colossi and Dragons. You just have to make sure your dwarves working outside actually stay near your traps - a fisherdwarf who goes wandering screens away from the nearest trap is not protected.

War dogs can also be assigned to dwarves who go outside frequently. Then when the dwarf encounters danger, the war dog runs at the danger while the dwarf runs away from it. Unfortunately, war dogs can't be reassigned once they are assigned. To get around this, have the dwarf you want to be guarded train the dog. (Dogs follow the one who trained them until they are assigned.)

Finally, a sufficiently large military could be used as a reactive force to rescue ambushed dwarves. The disadvantages are many - soldiers must physically move to the conflict zone which may be many screens away from the nearest entrance to your fortress, by which point dwarven lives may have already been lost. And while squad organization may make ordering a large army easier, a squad commander who is sleeping, eating, or drinking prevents his entire squad from responding. At best, an army should be considered supplemental for defending dwarves outside your fortress.

Armed Civilians (Gobbo Season Open!)

It can be a fairly decent idea to keep mass numbers of cheaply-made crossbows (or your lower-quality rejects) and bone/wood bolts on hand, and all expendable dwarves in one mass military squad set to use crossbows (and leather armor, if you have enough). What dabbling marksdwarves lack in speed and accuracy, they more than make up for with incredible enthusiasm, as a hailstorm of pathetically-aimed bolts will tear over anything stupid enough to move. Not nearly as effective or useful as properly-emplaced marksdwarves with high skill and proper equipment, but a good emergency measure, especially if you keep your craftsdwarves busy churning out cheap ammo from spare bones from the kitchens and cheap crossbows from fishbones from the dining hall.

non-hunting hunters

Sometime you will embark in an area devoid of (huntable) wildlife. In that case, you can turn on the Hunting skill for all civilians and use the military menu to arm (and more importantly, armor and shield) them. Normally turning on hunting will cause dwarves to wander outside looking for wildlife, and turning it on on all your dwarves would delay your economy greatly - but without wildlife, no hunting jobs are generated, and they go about their business armed and armoured. Note that if X number of hunt-able animals do appear on the map, that many dwarves will then go hunt them.