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[Hydra]

My current for has an 3-tile wide entryway which first passes a bunch of weapon traps. They all have either 10 obsidian swords or 10 glass serrated disks. All trap mechanisms are of the best quality. Then the entry road goes past a shooting gallery where my markdwarves can shoot at them from above. After that they are 'inside' my outer fort. I can pull up the drawbridges to my inside fort, which means the gobboes have to go into a cistern like structure, down 4 stairs into a winding path of muddy obsidian tiles. When the whole group is down there, I pull up the bridges into the cistern, and open the first set of floodgates that dump craploads of pressurised water from my water tower into the chamber (the first test showed that the chamber didn't fill up instantly so I made the tower bigger so that it will). If some gobboes would make it past THAT, I still have 12 champion hammerdwarves lusting for blood.

So you wonder, how did my first siege go? Well, I got a group of goblins. They ran into a weapon trap, which instantly splattered 3 of them all over the area. The rest went "Hmm, lets not" and I haven't seen any gobbo's since

[LrZeph]

Ballista and an 11 z level drop, ain't ridiculous but the corpse explosions are.

[Sphalerite]

Three tile wide paved ramp coming in from the map edge.  Past the initial guard dogs the passage widens, then divides into three three tile wide corridors running side-by-side.  Corridors are floored with bridges, thirteen 10X3 bridges in each, over a ten Z-level-deep drop.  Alarm dogs are chained in alcoves in the walls to spot thieves.  Then the corridors rejoin, pass a final set of barrier bridges and alarm dogs, and then enters my trade depot.

When I am expecting a caravan, the system is idle.  Trade convoys spawn at the entrance and make their way down the central isle to the trade depot.  But because ambushes seem to spawn at about the same time, as soon as the trade convoy is in I activate it.  I learned the hard way that having a defense system that requires half a dozen widely scattered levers to be pulled in the right order is a bad idea.  For this fortress, a single lever in the center of the meeting hall controls the entire defense system, and I always make sure to have an idler ready to pull it.

Pulling the lever opens a floodgate deep in the bowels of the fortress, letting water flow out of base of the ten level deep cistern.  The rivers, my only source of water, freeze during winter, so I made sure to keep enough water on hand to run the defenses for several years if necessary.  The water flows into a complex system of pressure plates, floodgates, and bridges that controls the entire defense system timing.

First the barrier bridges closest to the trade depot retract.  Those bridges are to prevent any dwarves, traders, or animals from wandering into the defenses while active.  They're retracting over a one-level-deep channel, so anyone on them when they open isn't hurt and can make it back to the trade depot chamber.

Then barrier bridges raise around the guard dogs at the entrance, and a one tile wide side path opens up.  I got tired of having to replace the entrance dogs every time an ambush came by, so when the defensive array is in kill everything mode I seal them off and shunt traffic around them.

Then the main system engages.  Each of the three corridors' bridge floor arrays open and close in sequence.  The system is set up such that at any one time at least one of the bridges will be solid and offer a valid path to the bait dogs at the other end.  The timing of the system is slow, taking over 600 cycles for all the bridges to run through a full on-off cycle, but at 130 tiles long nothing's made it all the way across.  The ten Z level drop is explosively fatal to everything that I've seen so far.  I have yet to have a siege or megabeast, but the system has killed ambushes, babysnatchers, kobold thieves, and random wild animals.  Someday I'll get around to cleaning up some of the bones, armor, and weapons littering the channel under the bridges.

While the defense array is running, my dwarves will be trading with the convoy, blissfully unaware of the carnage going on.  By the time I get the "Traders will be leaving soon" message the entrance is generally clear.  I send a dwarf to pull the lever again.  The floodgate from the cistern closes, and drain gates open up to drain the water from the control logic.  The main bridge arrays stop cycling and go solid.  The barriers around the guard dogs open, and only when everything is safed will the safety bridges near the trade depot allow passage.

This fortress has no military.  I have only had two deaths so far:  a legendary miner, one of the original 7, who died in an unfortunate accident while digging out my cistern, and an immigrant hauler who insisted on going for a stroll outside and got ambushed.

(edited to fix math error)

[Incendax]

Just a slight variation on a classic method.

The entrance to my latest Fortress has a walk-in area that is 10x40 and 3 Z-Levels high. Two Levers operate the Outer and Inner doors, respectively.
The bottom Z-Level is where the besieging goblins walk in. The middle Z-Level is nothing but Floor Hatches that are rigged to a single "Doomsday" Lever, that also attach to the Outer and Inner doors.
The top Z-Level has a 8 Screw Pump station attached to the River and operated by Waterwheel.

When the goblin siege walks the majority of it's force into the 10x40 I throw the "Doomsday" Lever and both the Outer and Inner doors snap shut, and all 400 Floor Hatches snap open to dump two entire Z-Levels of water right onto the goblins heads. They shortly drown, and any stragglers left outside flee off the map. I then hit the Lever to the Outer doors and all the water rushes out into the natural world to eventually evaporate.

Bonus points if you pave the entire 10x40 so it doesn't get muddy. 

[NFossil]

I just tend to use deep pits below the entrance bridge, with a water or magma reservoir directly above, and grates+channels at the bottom for draining/magma recycling.

[blackfire83]

...all 400 Floor Hatches snap open to dump two entire Z-Levels of water right onto the goblins heads....

Wouldn't 4 bridges do the same job, with far less construction and far fewer mechanisms?

-blackfire83

[Zaranthan]

Yes, but 400 hatches is SO MUCH COOLER.

[Incendax]

It's more Dwarfy when you've got more than 100 of anything.

Like that time I built a Fortress on the Desert and covered the entire ground Z-Level with Stone-Fall Traps. That was over 16,000 I think.

[HollowClown]

I once had an entrance that passed directly through an enormous stockpile of forbidden booze.  When the invading goblin army tripped a pressure plate in the center of the booze, it released four caged fire imps into the corners of the room.  The fire imps threw fire balls at the goblins, the burning goblins set fire to the booze, the booze exploded, and the resulting firestorm killed over 3/4 of the dwarves in the fortress.  The fire imps lived.

[Glory of Arioch]

The defense mechanism I use for my forts involves computing, steel spikes, and a lot of mechanisms.

The main entrance to my fort:

Spoiler (click to show/hide)

Two bridges are used to control access to the fort. The purple one, if withdrawn, bars all access to the fort. The yellow bridge, if retracted, forces invaders to take a winding path one Z-Level above the fort.

One z-level above my entrance:

Spoiler (click to show/hide)

Invaders enter via the purple staircase. They then have to travel around the winding path of constructed floors in order to access the yellow ramp, which leads down to the fort. (In the screenshot the trap is off, so the grates have been deployed to allow easier access for trap cleaning. When the trap is on, the grates disappear.) When an unfortunate invader steps on the pressure plate, it arms a memory cell in the logic cistern of my fortress. This memory cell operates one of the inputs of an AND gate. (The other input is a System ON/OFF lever, which allows me to deactivate the trap for cleaning.) The AND gate operates a clock cell, which, when activated, cause the 80 retracting spike traps (10 steel spikes each) on the catwalk to repeatedly deploy, skewering any passersby. Once the invaders have retreated, the CLEAR input on the memory cell is pulled, the trap deactivates, and my shoe-lusting dwarves rush in to claim their new footwear.

I used to have a dragon guarding the yellow ramp, but he was killed by the titan (visible in the first screenshot in the cage.) Also, the entire catwalk is within range of my marksdwarves battlements, but I've not had to employ them to repel any invaders, because the traps seem to do the job nicely. Also, my trade depot is outside while I debug why my Depot Drowner is leaking water.

[Hyndis]

A long closed corridor over a magma reserve that i pressurize with pumps when the gobs enter the corridor. the pressurized magma rises one level and then sinks back under the corridor and leaves neatly stacked piles of iron.

pretty elegant indeed.  using grates for the  corridor floor?

In a freezing environment you can also do this with water. Have the corridor be made entirely out of downward stairways. Dig out the ceiling so the corridor is exposed to the cold, but the area below it is still considered indoors and thus the water will remain liquid. Pressurize the water and then open the floodgates. The water will try to equal out its level, thus swelling up from those stairs and instantly ice cubing the goblins.

 

[Hyndis]

Then I set the lever controlling the drawbridge on repeating Pull.

Why not use a pressure plater instead of the lever on repeated pull?  If you do though, I advise building a bridge before *and* after the pressure plater, as there is a lag after they step on the plate, and so the back ones don't just get to run away (muhaha)

Hmm.  I hadn't thought about that.  I'll have to research pressure plates and see what triggers them.  (I'm too nice - I let the elves live.)

The best way for an automatic system is to chain up a kitten. Then surround it with pressure plates. You can do this by limiting the space it can roam in to just two squares, with one having the chain and the other having the pressure plate. Link the pressure plate to whatever you want. Then as the kitten wanders about, it will continually trigger the pressure plate, thus slaughtering entire armies of goblins.

You can chain up more kittens, linking more pressure plates to the same mechanisms to have it triggered even more often.

Or you can make a spiked floor, with upright floor spikes, but have, say, 6 kittens and each kitten linked to 1/6th of the spikes. You now have spikes that randomly shoot up out of the floor.

 


Kitten powered death machines are always amusing.

[ShadeJS]

Sure I need to construct a new one every time I use it, but there is something fulfilling about the dust cloud that results from the dropping.

It's good for the economy...

[Aspgren]

A huge tower resting on a single support. The goblins have to climb up the tower, criss-crossing from side to side and going through a small maze so it takes an incredibly long time for them to enter. Once there are alot of goblins in the tower a lever is pulled and they all come tumbling down.

Not effective at all I'll admit but it's pretty fun... once you've built the damn thing.

[Slappy Moose]

A big, cylindrical chamber with a maze of catwalks and spiral staircases going around the perimeter of it. At the top is a hole, where water is poured through.

Goblins must race the rising-water level to the exit at the very top.