I don't like to do things too gimmicky,... and yes, I'll probably pay for that.
But I've got a large, walled surface compound, with a moat around it. Traders enter over a bridge (not too far from the edge of the map, but not so close as to be a game exploit). They then travel between two walls for more than half the width of my compound, cross another bridge (a retractable one, this time), then turn left to arrive in my trade depot.
If the traders are under attack, I can close the first bridge temporarily, to shield them from arrow fire. Then, I'll let goblins over the first bridge, then close both bridges. That will make a shooting gallery for my marksdwarves on the level above, shooting through fortifications. And since that second bridge is retractable, I can also set up siege engines in the far corner of my compound and shoot down the length of that goblin trap, too. (So far, I haven't had time to work on siege engines.)
That's the plan, anyway. I've heard that Elite goblin bowmen will decimate my marksdwarves, right through their fortifications. (I've had goblin bowmen, but apparently they weren't Elite.) But it seems a little more fun than just setting traps, though I
have had traps while the fortress was under construction (and I still have them at the north entrance).
Building destroyers won't be able to get to the bridge, over the moat, when raised - not unless they can fly. Well, flying enemies will be a huge danger, anyway. And if I'm under siege, caravans won't be able to get out. I suppose I should make a second, secret entrance, and only open the bridges to that in an emergency.
Of course, problems like that are
fun, right?