How can such a little thing, all alone there in a sea of black, be a problem, you may ask?
What am I actually looking at? you may also ask, if you don't play the game.
That's the central staircase. A tunnel leads to this staircase, south of the slums of Stesokur. It's a shaft that leads straight down into the center of the earth. From this shaft my miners dig out in various exploratory directions, seeking ores and gems and precious stones. The shaft also leads directly into the caverns, wherein various monsters dwell. It's an easy thing for giant crocodiles and lizards and whatevers to just storm right up those stairs and into the fortress. I mean, yeah, they attack the slums before the dwellings of the important dwarves, but still. Something needs to be done.
So I made some modifications:
"It looks like a penis!" said my girlfriend. "Certainly not my penis," I responded. I think she's doing it on purpose, now.
It may be a little difficult to tell what exactly I did, especially to those who don't play the game, so let me describe the situation:
The bottom of the second picture there is where the Central Shaft used to be. Technically it still goes all the way down, but I've changed the stairs. There is now only one set of stairs leading upwards, and none leading down. The old stairs, which used to go all the way down to the caverns, have been floored over. So now you can no longer go down the central staircase. Instead, you have to go north - through a narrow tunnel filled with cage traps.
Once you've gone through this tunnel, you reach a 3x3 chamber filled with stairs that go down. Once you've gone down a single level, you're faced with another tunnel, heading south, back into the shaft. That way all of my dwarves - and every other creature - that want to pass through the Central Shaft need to take a small, trap-filled detour on the way.
If you don't understand what I just wrote, don't worry about it. That's because I'm a bad writer, not because you're a bad reader. Let my expert MSPaint skillz do the talking. Here's the detour, viewed from the side:
My degrees in philosophy and law may be as worthless as an art degree, but at least those guys can fucking draw.
See how it's a straight line of stairs heading all the way down? And see how there's a floor forcing you into the tunnel to the left? That floor was built on the stairs to stop things from coming up. If you want to go down those stairs, you have to go left through that thin tunnel, past all those cage-filled traps, down another flight of stairs, and then through another tunnel, to the right, which takes you back into the Central Shaft. More importantly, there's the same detour going up. Any monsters or dangerous critters can't just run up the stairs... they have to make their way through dozens of cage-traps first.
This should take care of anything getting into my fortress from below. So I'm now well-protected from all threats; those above-ground, and those-below.