I just recently got my main fort ready, and I established a drop-bridge accessway to the place.
Construction just started for the main fort, but the bridge is built and linked. I tested it, and it's fatal enough to be effective (Pic related; poor bastards didn't see it coming). We get occasional survivors; perfect for an arena to wait below. In case they prematurely land (missing the arena), they'll have to work their way into the arena through a trap gauntlet to soften them up, since they didn't fall as far down as their buddies deeper down the bridge.
Got a marksdwarf pillbox on the side, and I intend to have a minecart railgun established firing from the front door, towards the ballista firing system opposite of it which also catches the runaway minecarts for rapid-reloading. Below the suspended fortress lies a barracks/arena. Since I'll also have a cistern lying below, I may need to have a way to protect it from any flying corpses and such, along with a method of drainage that doesn't compromise the rest of the land. Just in case.
As I work on this, I also cut out a chasm 5-tiles away from the edges of the map, and setup drawbridges on various points to re-route invaders and trade caravans, with a decoy trade depot so I will always receive wagons, but even I can alter whether they go to it or the primary depot across the drop-bridge. I have a burrowed dwarf in a switchbox room I have specially made for them (living quarters and dining room for them) so they're always at my beck and call. In case they sleep on the job, the room can still be accessed by everyone else, including those who keep their food and drink in stock.
EDIT:
Oh yeah, I also have my fort built along the walls of a volcano (cheap heating, if you will). Through modding, I can always make sure it's raining out, and let my invaders through the front door, and watch them melt before my eyes, making the core of my fort a microwave (that bug was awesome). Alternatively, I can always include a magma dump zone; but I already engineered enough ideas in my head; doing so, efficiently, and keeping the other parts functional as their design intends, may be a challenge. The minecart railgun would be compromised, along with the barracks/arena below.