I've been considering channeling out the area around my fort, but leaving ramps going from the edge to the middle. This way my entrance is at the bottom of my fort, and ideally also located between my fort and the caverns. This means i can have a single military area for both above and below ground threats. I am having trouble taking water sources like Brooks or rivers into account. One easy fix would be to dig around them, but I'm not sold on that idea. Any thoughts? What external fort design do you use?
Here's my mental image of what your fortress could look like with a re-routed river or stream.
First, block the river or stream. Don't do it right at the edge or you might destroy the water-creating tiles.
Channel out several layers in a square around your fortress, going far enough out from your fortress to allow for a four-tile-wide river course to be cut and pass under your ramps. The ramps suitable for caravans are going down on all four sides of your pit.
Now you channel a four-square wide ditch all the way around the outer edge of your pit, except where you go under the ramps, where you dig a tunnel. On the side opposite from the blocked river or stream, dig to the edge of the map and make fortifications for a drain. Unblock the river or stream. You should end up with a waterfall, leading to a river which drains off the map. You might want to make the riverbed two tiles deep in case it might flood otherwise.
I made a very badly executed sketch in Paint, but I just discovered that my FTP program doesn't work under Windows 7. *sigh* So, you'll have to imagine the illustration until I can get something to work for file transfer.
Edit: I got it! I had to boot up my WinXP Pro machine, but I got the sketch uploaded. Like it said, it is badly executed, but you can see what I am picturing. The fourth ramp, on the side of the viewer, is left off for convenience.