I've developed an entrance style that has worked out really well for me. It will effectively stop all invaders and will break most sieges fairly quickly. It allows wagons to reach a relatively safe trade depot that is virtually impervious to thieves. It also allows for marksdwarves to defend the fortress with violence should that be necessary or desirable. This will not necessarily suit everyone's play style, as it relies on cage traps and a hatch cover to ensure the fortress's almost complete safety. Still, I'd like to share it, as it's quite simple and relatively space-efficient.
Here is an image of the basic design. I apologize to those of you who prefer using fancy graphics packs. I learned to play dwarf fortress totally vanilla, and now I'm so used to it that using graphics packs is like trying to learn a whole new game.
Why I call this the snail shell entrance should be rather obvious. The entrance curls around in a spiral, like a snail shell, with the most vulnerable part in the center of the spiral. You'll notice the single up stairway just to the right of the trade depot. That is the only entrance into the fortress. One level above, you place a hatch cover, and from there, you can pretty much design your fortress however you want. I usually dig out a hallway and line it completely with cage traps made from high-quality mechanisms. This serves the dual purpose of providing a safety mechanism in case I accidentally allow invaders to get past the hatch cover and improving the moods of my dwarves.
Now, let's discuss the main features of this entrance, and how it works. The shape is important. It can curl either to the left or to the right, but it must always curl in the same direction. The reason for the curl is so that you can place cage traps along the inner walls to stop invaders.I prefer cage traps because a trapped enemy is taken out of the fight immediately and can be safely stripped and either thrown into the nearest volcano or used as a training dummy for military dwarves. You'll notice that I only have one layer of traps placed right now, and that my cage traps have already caught a number of invaders. I always carve out enough room for a second layer of cage traps, though the effectiveness of this second layer is rather marginal, and so I hadn't gotten around to it yet. The lengths of the hallways or the inner chamber is not set in stone. Longer hallways will provide better defense, as invaders will more closely follow the inner walls of the spiral the longer the hallways are. You could even expand the spiral, adding more turns to increase this effect even more. However, the design as I show it here is quite effective, and I think adding longer hallways would only slow down dwarves that are trying to enter or leave the fortress, and increase the wait for caravans to arrive at the depot.
The floor hatch at the end of the spiral is important, as it serves as a stopgap measure in case invaders are able to get past the cage traps, either because they are immune to the traps or because they managed to simply overwhelm them with sheer numbers. Building destroyers cannot deconstruct a floor hatch from below, meaning that once that hatch is locked, it is impossible for anything to enter your fortress. I prefer a floor hatch to a drawbridge as it can be locked instantly, with no input from your ever-fickle dwarves required. The hatch can be made very quickly out of either wood or stone, and doesn't require any additional steps to make it functional like a drawbridge would.
You'll notice the fortifications along the outer walls of the entrance. These allow marksdwarves to fire on invaders in relative safety once the hatch has been locked. This will often break a siege if your cage traps weren't enough to do that already, and is pretty much the only way to deal with titans short of sallying forth to engage them head-on. Obviously if the titan can breathe fire or spit acid, this strategy won't work as well.
Now, obviously this fortress design is far from absolutely perfect, and several improvements could be added. For example, drawbridges could be placed in the hallways. Once set up, they could block off any path except the one that heads through the cage traps, further funneling enemies into them, and smashing any enemies that evade the traps. Further layers of defense could be added, such as enclosed watchtowers or bunkers outside the fortress. War animals could be penned or tied in the hallways or just outside the entrance. You could even set up siege weapons to cover the hallways, or turn the entire thing into a drowning chamber. This is Dwarf Fortress. The possibilities are endless.
One advantage of this design is that it is modular and can be expanded upon as your fortress grows and develops. Digging out the spiral and setting up the floor hatch works quite well as an early line of defense. Cage traps and fortifications can be added later, once you have enough materials and labor to build them. Also, this design is very useful for laying out the action in a way that is easy to see and follow. You can set a hotkey to zoom you directly to the entrance and see which dwarves are still straggling to get into the fortress and how close the invaders are to reaching the door. This allows for relatively precise timing for when the floor hatch is locked, ensuring that any dwarves caught outside when the attack comes have the best chance of getting inside before the fortress gets locked down.
So, what do you guys think of this design?