A few notes on my fortress design:
You may wonder why my entrance corridor is so wide (five tiles), so far from the main stairs, and bends through a right-angle. It's awkward in the short run, but is all part of my LONG TERM design for our defenses.
I believe in using LAYERED defenses. Here's my evil plan:
The outermost defensive layer... the one that invaders will hit first... will be an array of Magma Mines: holes in the ground that lead down, across, and then UP a couple of dozen levels into the air (that latter part must be constructed, of course, not dug) into an overhead Magma reservoir outfitted with a large number of pumps. As invaders enter the map, we flip a lever to turn on the pumps, thus pressurizing the Magma in the conduits. As they approach the array of Magma Mines, we flip a second lever to open the floodgates... shooting pressurized fountains of Magma hundreds of feet into the air, to rain down on the beardless heads of our enemies.
The next defensive layer is a fortified outdoor compound... a high and thick wall, with Fortifications along the inner and outer edges, and with the inner compound roofed-in to prevent aerial attack; with fortified MarksDwarf towers at strategic points such as gates and corners. There would typically be two openings in the outer wall... the first one a narrow and heavily-trapped path leading over a very (very) deep pit, which would be left open pretty well all the time; and the second one a seven-tile-wide wide air-lock (with three-tile-wide inner and outer drawbridges) which contains our trade depot. This air-lock would usually have only the outer door left open... and when a Caravan enters the map and arrives at the depot we would shut the outer drawbridge and open the inner one, so that we could carry our goods out to trade with them. Caravan wagons will NOT cross traps, so all of our traps will be located just inside the INNER air-lock drawbridge... the Caravan itself would stop at the depot inside the air-lock; so it would not need to cross the traps. I'm planning (later) to hollow out a large underground room directly beneath the outdoor compound and fill it with Magma, so that our entire outdoor compound has "central heating" from the Magma immediately below it. This will allow us to grow outdoor crops such as Sunberries and Strawberries.
The next defensive layer is built into the ground-level entrance (and hence the odd design of my entrance corridor). First a five-by-two drawbridge to act as a "shutter" that can be closed to cover our doors. This shutter would be closely flanked by 1x1 "walls" which prevent building destroyers from attacking the left and right edges of the drawbridge itself. In effect, the drawbridge "shutter" would be counter-sunk into the hillside, with only its outer surface (the outer surface when shut... the lower surface when open) exposed to approaching enemies. That makes it totally immune to building destroyers WHEN IT'S SHUT (but not, of course, when it's open).
Then two pairs of doors, with an unmined 1x1 wall between the second and third door. Doors MUST be set up next to a wall, so the arrangement wall - door - door - wall - door - door - wall gives us a five-wide entry-way containing four doors; with all four doors properly supported. These doors can of course be locked when we don't need access to the outdoor compound. Then a row of traps, just inside the doors.
The five-tile-wide entrance corridor (accessed by the doors) will be flanked on both sides by ditches, which will prevent any opponents who make it that deep into our layered defenses from getting adjacent to the Fortifications on the opposite (outer) side of the ditches. To the left and right of the entrance corridor (and separated from it by the ditches) will be fortified rooms for MarksDwarves, who will also be able to shoot out through other fortifications at anyone crossing the outside compound. Where the entrance corridor turns through a right-angle (to head towards the stairs) will be another ditched-and-fortified room containing three Ballista, set back from the walkable path far enough that the operators won't be frightened away by the approaching enemies... siege-weapon operators are counted as civilians, not military, and will run if the enemy gets too close. The ballista will be positioned to fire down the length of the entrance corridor... and out into the compound if desired.
The last line of defense will be another set of drawbridge-shutter and doors; so that if the whole situation falls apart we can seal ourselves off again by conceding the outer defenses.
Naturally, we can incorporate optional Magma into this defensive plan.
