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I have a 3 wide hallway leading into my base up to the Trade Depot, in the book I was reading this is how I was told it would be accessible to caravans. However, this wide entry way is allowing monsters such as a recent group of Giant Flies to get in and interrupt my dwarves as they travel around doing various tasks.

I assume that if I block off the entrance and put a door, the trade route would no longer be wide enough for a caravan.

So how do I protect/secure the entrance yet still have it open for trader events?

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This question is unfortunately broad. There's many many many methods. –  Daenyth Jul 29 '12 at 23:56
    
Have you tried a bridge? Make it so when it is in the up position it blocks the hallway. But, you need to put keep it down normally, else, without a trade depot the merchants will skip your caravan. Another way might be, make the corridor 5 wide, and put a line of traps on the edges. Not ideal, but with some luck any invaders walk into the traps first. –  Ids Jul 30 '12 at 2:50
    
The most effective (and therefore boring) way to defend anything is to wall it off, but that doesn't work for depots as they need to be accessible from the map's edges. The second most effective method is traps, thousands of them. –  kotekzot Jul 30 '12 at 9:34
    
Traps are nice. They add to your fortress value and provide your mechanics with valuable skill training, plus they're a way to put superfluous weapons to good use. –  Shadur Jul 30 '12 at 20:41
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3 Answers

As mentioned in the comments, bridges are useful -- keep them lowered normally, but raise/retract them during sieges or when an ambush is detected (On that note, patrols and watchtowers help a lot to spot an ambush before it knocks on your door).

Another major help are staggered walls -- a few twists and turns in their path won't annoy the traders unduly, but breaking line of sight early and often makes enemy archers much less threatening (given that their ranged fire tends to amount to 70% of my total casualties on average if I don't make sure I can take them out of the equation, this is a big consideration).

On that note, you can significantly improve your fort's security by keeping a few facts of the AI in mind and mechanics in mind:

  • Its pathfinding routine is essentially clairvoyant when it comes to routing, but it doesn't see traps;
  • This means that it will choose a trap-studded one-tile-wide suspended floor path that leads almost straight into the fortress over an open corridor that's significantly longer to walk through;
  • A trap only becomes "stuck" if it fatally injures something -- IE, when the wound is immediately fatal. A trap that 'merely' removes a limb or cuts through enough tissues to cause fatal blood loss won't need to be cleaned for the next victim to come across it;
  • An enemy that makes its reflex saving throw and dodges the trap automatically moves into an adjacent untrapped tile -- even if the only tile matching those criteria happens to be over empty air;
  • Once you have magma furnaces going, and an on-map source of sand, green glass upright menacing spikes and green glass serrated discs are essentially free.

Here are a few ways to bring these facts together in fun and interesting ways to cause many, many enemy fatalities:

  • The Long Walk Off A Short Pier: It's not a proper fortress until you have a decent moat along your walls, so I'm taking that as given. You don't need to fill it up yet, either. Aside from the wide drawbridge-covered main entrance, build one or more one-tile-wide floor paths over the moat, then build a one-weapon trap on each tile... And built upright spike traps on each tile to either side of the walkway in the moat below. Result: When a goblin manages to dodge the trap, he'll "dodge" right off the side and onto the spikes below. If the fall doesn't kill him, well, having made sure that the only upward slopes are on the outer edge of the moat means he has to climb back out and try going over that trapped walkway again.

  • Circular Reasoning The pathfinder AI instantly detects any changes in pathing due to doors opening and closing, so if you have a trap-filled corridor that has two gated entrances into your fort, and a set of pressure plates near each gate that causes the near gate to close and the far gate to open when enemies step on it... Let's just say it can keep your uninvited guests entertained for ages. Or until you run out of guests.

  • Flushing: This one requires a decent amount of power and a lot of pre-planning. Build a large reservoir of water (or for advanced mad scientists, magma) several Z-levels above the river/creek. Use built walls, floor grates and dug channels to make sure the outflow pours into one end of a long corridor and out the other, then rig up pressure plates to link to floodgates that seal the corridor and open the reservoir when an enemy steps on them. This setup is definitely for advanced mad scientists, as minor mistakes in planning can very easily lead to flooding your entire fortress by accident. Remembering the game's motto is recommended.

The wiki has a large page on trap design. Including a few good ways to potentially increase fun. My advice, make a flamethrower bunker.

EDIT: Almost forgot; you don't need to have the trade depot inside the fort proper; just be sure to secure the perimeter -- it's a diplomatic faux pas to let raiders destroy friendly caravans.

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You've hinted at dodge and fall traps, maybe you should elaborate a bit. –  kotekzot Jul 30 '12 at 8:11
    
Done. Like I said, I can't get to df.magmawiki.com on my work machine, so I can't link directly to their article on traps. If you can check and add the links, I'd appreciate it. –  Shadur Jul 30 '12 at 8:51
    
I think you could also put a few war dogs in a zone in front of the depot. That should also stop most minor incursions. Perhaps you can add this to this answer. –  Ids Jul 30 '12 at 12:14
    
War dogs are excellent against sneak thieves and snatchers, but if there's more than a single bowman in the attacking group and they have a clear line of sight your dogs'll be dead before seeing any action. –  Shadur Jul 30 '12 at 17:08
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@shadur, df.magmawiki.com redirects towards dwarffortresswiki.org –  Ids Jul 30 '12 at 17:23
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There's great article on security design on the DF wiki here. The final section has a great design that I've started using for all my new fortresses. Not foolproof (I am an ingenious fool!) but a solid place to start from.

Features:

  • Easy access to depot via bridge (when lowered)
  • Long, winding, trap-filled route for enemies when bridge is raised
  • Pit catches enemies that fall off said route, forcing them back to the start
  • Long paths for ballistae to fire down and impale multiple enemies
  • Fortifications allow archery squads can fire at intruders
  • Chained war dogs to detect sneaking enemies

Full layout here, hidden under spoiler for people who want to work out a design on their own:

                           Base
                           ║╞═╡║
                           ║D+D║ ╞═╡= Bridge
                           ║D+D║  D = War Dog (chained)
      ☺  ☺  ☺  ☺  ☺  ☺  ☺  ║+++║  ^ = Trap
    ╔╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬═╝+++╠════
    ║.......................+++║▐▀\
    ║.++^+^+^+^+^+^+^+^++++++++╬◄═«
    ║.+.....................+++║▐▄/
    ║.+.....................+++║▐▀\
    ║.++^+^+^+^+^+^+^+^+++..+++╬◄═«
    ║....................+..+++║▐▄/
    ║....................+..+++║▐▀\
    ║.++^+^+^+^+^+^+^+^+++..+++╬◄═«
    ║.+.....................+++║▐▄/
    ║.+.....................╞═╡║▐▀\
    ║.++^+^+^+^+^+^+^+^++++++++╬◄═«
    ║.....................+++++║▐▄/
    ║....................>+++++╠════
    ╚════════════════════╗+++++║
                         ║╞═══╡║
                        Entrance 

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Just a quick warning for the suggested layout: enemy archers well be able to fire through fortifications on the same z-level, and civilian dwarves will flee from the ballistae if the invaders come too close. –  jaminja Jul 30 '12 at 15:06
    
Fair points. Your archers should have the advantage as they're much closer to the fortifications, but yes there is a risk of injury. The wiki seems unclear about when civilian siege operators might flee (suggesting "as far as 10 tiles", then "around 20"). Perhaps move the line of ballistae back a few tiles to give them a bit of extra room. –  raveturned Jul 30 '12 at 16:05
    
I also don't see a reason /not/ to place traps on every single tile of the path there. Especially as you can make the floor below a spiky mass of death... –  Shadur Jul 30 '12 at 17:15
    
@shadur A retractable spiky mass of death of course. As added bonus, add possible water to the floor below. Drown any survivors. (When finished, drain the water away, and get the goblinite). –  Ids Jul 30 '12 at 17:22
    
Can never have enough Goblinite. I've had maps where that was my only reliable source of iron. :) –  Shadur Jul 30 '12 at 20:39
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I use draw bridges to create a protected area. I place the trade depot inside my fortress, with 1 draw bridge between it and the outside access way.

I then place a second draw bridge between the trade depot and the rest of my fortress. This is because traders and their animals seem to get impatient when a seige is outside my fortress, and like to go mad. By closing up the inner drawbridge between the inner fortress and the trade depot, I can drop the lower draw bridge, and allow the trader caravans that want to risk passage through the seige forces the opportunity to try it. They usually decide it is a bad idea--- just as they get killed. But then it is too late, and I get their goods as a prize for having to clean up the mess they made.

Be sure you place your draw bridges so that the wall made by the raised draw bridge faces out towards where you expect the dangerous things to be. This is actually necessary to keep tantrumming dwarf traders from destroying your inner gate. Obviously, you just drop the outer gate when the dwarf traders get that antsy and let them try to outrun the seigers.

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