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.

• + - fortification
•   - pit (filled with spikes, water, magma, whatever you prefer)
• . - Floor
• 1 - Bridge 1 (over pit)
• 2 - Bridge 2 (over pit)
• 3 - Bridge 3 (over pit)
• # - Wall
• r - Your ranged stuff here

   
                  ENTRANCE
######################222######
rrrr+ ................... +rrrr
rrrr+ .               111 +rrrr
rrrr+ .  .   .   .   .... +rrrr
rrrr+ . . . . . . . . ... +rrrr
rrrr+ . . . . . . . . ... +rrrr
rrrr+ . . . . . . . . ... +rrrr
rrrr+  .   .   .   .  ... +rrrr
rrrr+                 ... +rrrr
rrrr+++++++++++++++++ ... +rrrr
rrrrrrrrrrrrrrrrrrrr+ ... +rrrr
rrrrrrrrrrrrrrrrrrrr+ ... +rrrr
rrrrrrrrrrrrrrrrrrrr+ ... +rrrr
rrrrrrrrrrrrrrrrrrrr+ 333 +rrrr

The 3 tile wide lane is for traders, so if you have your trade depot before this, cut it down to a 1 tile lane to slow down invaders more. Retract bridge 1 to force invaders to take the long way, for maximum exposure to your ranged units. Bridges 2 and 3 can be put down to allow more enemies into this pit entrance, then retracted again to trap them again.

The "Inverse Battlement" Design by Sergius

Note that the fortress interior is to the West. The enemy forces come from the East. One by one, they fall as soon as they pass this "bridge" (which is one level above), as the marksdwarves wait in ambush. For extra safety, hollow this tunnel out to the mountain, upwards into the sky (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 specially nasty, make the tunnel really long into the mountain and build a ballista behind some fortifications. 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.

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.

Siege weapons

One effective way to have seige weapons defend your fortress is:

• . - floor
• # - wall
• s - ballista
• > - Stair
• - channel

        ###########################sss
Entrance...............>     sss...sss
Entrance...............>     sssssssss
Entrance...............>     ssssss...
        ########################sss###

Using this design you can cripple an army using a well timed volley.

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, afterwards 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.

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 dwarf 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.