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[Dwarf4Explosives]

Well, being a bit of a newb when it comes to dwarf fortress, I've decided to see if I can come up with a system to stop raids. Basically, it's a several z-level trench at the edges of my embark square, deep enough to kill basic enemies, with another ditch behind it to stop dwarfs from falling in.
The problem I have is that I can't think of a way to stop this from killing new dwarves coming into the fortress.
Here's a quick depiction of how I plan to construct said ditch system, with the numbers depicting the depth:
0000000000        0000000000        0000000000
0111111111        0111111111        0(10)(10)(10)(10)(10)(10)(10)(10)(10)
0100000000    -> 0100000000    -> 0(10)00000000
0101111111        0102222222        0(10)02222222
0101111111        0102111111        0(10)02111111

Any suggestions?

[jcochran]

Good luck on that. Trenching the entire perimeter of your embark is a quick simple way of stopping cold the first few sieges, ambushes, and what not...
For preventing access, a trench just 1 Z level deep is sufficient if its walls are vertical. Having a deep trench is just for the splat and kill effect. And frankly, unless the creature has to dodge something, it's not going to fall into the trench in the first place. The "not going to fall unless dodging" also protects your dwarves from falling as well, so you don't need a second "accident prevention" trench either.

However, you also need to take into consideration the need for the following items.

1. Migrants need to be able to path from the edge of the map to a meeting area.
2. Traders need to be able to path from the edge of the map to your trade depot.
3. Wagons need to be able to path from the edge of the map to your trade depot the instant they appear (e.g. You don't have time to lower a bridge or in any way react to the wagons appearing. If they can't path at the moment they appear, they skip your fortress entirely). This means you have to predict when they'll appear and take action. Or deal with the fact that you'll never get wagons. Or always have the depot wagon accessible.

The first two issues can be solved easily enough via draw bridges and such (although those aren't the only solution). But the issue with wagons requires a bit of sneakiness....

In general what you want to do is have "controlled" access from the edge of the map. Make the entire edge inaccessible except for places you determine  where you have an appropriate welcome waiting.
Things to note:
1. Migrants and traders will not set off traps. Nor will they set off pressure plates (unless 'citizens trigger' is enabled). In that case, migrants will trigger the pressure plates, but traders won't. So you can have the "welcome center" heavily trapped and don't have to worry about it harming incoming migrants and such.

2. You can dig a trench that wagons can cross, but nothing else that's ground bound can pass. So when a caravan appears, the wagons head towards your depot cross country while the guards and pack animals go around and pass through the welcome center. The trick is how ramps work.

Spoiler (click to show/hide)

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Z-1

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If you simply channel the boarder and then dig a tunnel next to the resulting ramps 1 Z level lower, you'll create a ditch that wagons can pass over freely, yet ground bound creatures can't. In the example showing, creatures arriving from the north or west, heading towards the south or east are able to go up and down the ramps, but are unable to get to the flat ground towards the southeast. So if your entire map boarder was ditched in that fashion, those creatures would be restricted to the two tile boarder around your map while the center of your map provides free access. And you can select where in that controlled perimeter you have a controlled access center to allow traffic you want. Creatures heading from the south or east towards the north or west, don't even have the ability to go down the ramp. So they're restricted to the upper level only.

[Dwarf4Explosives]

I mainly was trying to figure out a system that can be implemented before a fortress has access to pressure plates or traps. However, the second tip is very helpful.

[jcochran]

Well, I tend to embark with 10 each of copper and tin ore plus either coal or lumber to jump start my metal industry. While some of the dwarves are channeling the edge of the map, I have the rest building stuff for the "welcome" center. Doesn't take a whole lot to build some temporary workshops after all. After all, the crafters might as well do something useful while the miners are digging away. A smelter, and forge is all you really need. Although adding in a carpenter's workshop, mason, and mechanic is nice too. None of the named workshops have to be built inside. And building a forge early prevents unfortunate situations where a kea steals your anvil. When I was using a welcome center, I needed the following to fully equip it.

6 hatch covers. Either make from bronze, wood, or stone. Any of 'em work. What you use depends upon what you have available.
1 9x2 drawbridge (could use a 9x1 bridge, but I like to see if it's up or down without having to hunt down the control lever)
36 mechanisms (6 pressure plates to control hatches, 3 to control drawbridge, 15 for weapon traps)
15 weapons (assorted for weapon traps).

If using metal, the above takes 62 bars. Or 16 ores in total. But in reality, all except for the weapons get made of stone that's obtained while digging the border trench and the initial fortress rooms. That means only 15 bars or 4 ores (2 each of tin and copper).

And the welcome center is "functional" enough to handle unwelcome guests after just 2 hatch covers, 7 mechanisms, and 1 weapon. (just enough to make a simple 1 weapon trap goblin grinder). Doesn't take long at all to do that. The other hatch covers is to arrange for a "roach motel" that let's goblins enter, but not leave once they realize their trouble. And the other 14 weapons or so is just pure overkill to deal with weapon trap jams. I can have up to 14 individual traps jam and still have a functional trap. The drawbridge is to protect the hatch's from building destroyers (which don't come until after a few years, so there's no urgency in building it) and to provide alternative pathing down a cage corridor underneath the welcome center. But I'm not counting the mechanisms and cages used for that corridor since it's not populated until a year or so down the road. Experience has shown me that I can generally trench a 4x4 embark perimeter and have a functional welcome center before autumn of the first year. My first ambush typically comes during autumn of the 2nd year, so there's plenty of time. When I was trenching, my general "plan of attack" was
1. Immediately pasture grazing animals, dig out farm fields, have woodcutter chop down trees around the border (darn things cause problems while channeling. So get rid of 'em early. Since the miner is busy digging elsewhere, the woodcutter can finish finish the job before the miner gets there), build smelter and forge using rock purchased at embark.
2. After fields dug out, have farmer create fields and start planting. Meanwhile, dig out initial stockpile area.
(at this point, all the dwarves except the miner and farmer are busy hauling stuff from the wagon to the stockpile. Also have generally produced enough rock to build temporary surface workshops and have the mason/mechanic churning out stone blocks, hatch covers, and mechanisms, and trap components).

3. dig out "welcome center", then start trenching embark border.

While miner is dealing with border, pressure plates and whatnot is constructed and emplaced in the welcome center. However, I don't place the hatch cover on the goblin grinder until AFTER the trenching is complete. I do this since I can look at error messages while placing the hatch cover. If my cursor is in the trench and I see an error message "no hatch covers available" and when I have the cursor over the hole for the hatch and don't see that message, then all is good. But if I don't see the error message, then that means I missed a ramp or something that I need to flatten to make sure that nothing can walk onto my territory without going through the welcome center. So it's time to look closely again at the entire border and see where the leak is. Sometimes watching dwarves hauling lumber from the border can point out where the leak(s) are.

In general, the dwarves have pretty primitive conditions for the first year, but during that year, all the initial defenses have been built and then for the next few years, attention can be made towards luxuries such as bedrooms, grand dinning hall, etc. The trench and welcome center can easily handle sieges and such for the several years with no effort on your dwarves part except for having to haul away goblinite and corpse pieces.

Right now however, I've been embarking on terrifying biomes, so spending that much time on the surface is contraindicated. So I'm getting underground as rapidly as possible and not entrenching the entire border.

[cue]

3. Wagons need to be able to path from the edge of the map to your trade depot the instant they appear (e.g. You don't have time to lower a bridge or in any way react to the wagons appearing. If they can't path at the moment they appear, they skip your fortress entirely). This means you have to predict when they'll appear and take action. Or deal with the fact that you'll never get wagons. Or always have the depot wagon accessible.

The easiest way around this little issue is actually to just have another depot that's outside of your defenses. Just forbid it as soon as the wagons appear, and put down your bridge.

[Hague]

I use a double raising bridge over a gap for my depot. Inner gate stays closed, caravan comes in, outer gate closes, inner gate opens.

Also, using the trenching method, you can create a long causeway that you either trench or wall off to the edge of your map to your fortress gate. You can fortify a wall for archers by building fortifications that will allow you to safely fire into the causeway area.

My current fort has a surface entrance area that's about 28x28 with a Gate-Depot-Gate. Outside the depot is the elite barracks where ambushers and thieves are met with horrible death.

Arrayed around the surface are cage traps 5 with five spaces in between them for foiling ambushes.

Spoiler (click to show/hide)

Like this:
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It's pretty effective for catching most critters and spotting ambushes. Current biome is a savage cold broadleaf forest so I have lots of wood for cages and meat from training and domesticating giant birds. Hooray for 36 meat giant ravens!