Can you post a screenie/upload a map to the archive? I've been trying to think of such a welcome center and I can't figure it out from your description.
Main level. From left to right.
Goblin grinder, entrance room, entrances, bridge. Upper left of entrance room is stairs leading to cage corridor. Unfortunately, you won't get any reports when goblin thieves have experienced the grinder. But you will from time to time find corpses and their no longer needed equipment. Each of the six pressure plates is set to trigger on non-citizen creatures of any weight and are linked to the hatch cover next to each plate. The weapon traps are a mixture of spiked balls and menacing spikes. I don't want to mince the invaders since the dwarves have enough hauling to do anyway after a visitor. Also, I don't want immediately lethal traps to reduce the chance of a trap jamming with a body. I'm totally fine with letting 'em bleed to death and not have the trap actually deliver the killing blow.
1 Z level lower. Just a simple tunnel filled with cage traps. Originally, I had a much shorter passage, but when over 20 traps on that version got filled in a single siege, I decided that more is better....
Not bothering to show the surface, but I do channel, then floor over the grinder so it's considered above ground and in the light to prevent miasma. It's nothing special, but scary effective. The uncontrolled area is to the east, and the controlled area is to the west.
The small room is simply paved over with a road to prevent plant growth accidentally blocking anything. The passage underground is a simple tunnel that passes under the hatch covers and links to both inside the entrance room and the uncontrolled area. When the drawbridge is up it still provides a "path" to my interior, but in order to get there they have to go through a lot of cage traps and if they manage that, have to survive the goblin grinder. Not gonna happen..... The combination of ramp and tunnel on the uncontrolled side will allow a wagon on the surface to pass over it without any problems, but other creatures have to go into the welcome center. So the only thing that can get past it are flyers and trap avoid creatures. I'm thinking of replacing the goblin grinder with a more elaborate item with dual passage ways so trap avoid creatures like kobolds get to pass through a retracting spike trap corridor and non-trap avoid gets the grinder instead. But haven't come up with a simple solution that allows passage both ways and handles traders (unfortunately, they don't trigger pressure plates either. Even if you set them to be citizen triggered. This is a major problem since they'd be treated as trap avoid creatures and get to visit the spike traps. Not very friendly. And since they come with both on wagons and on foot, they are a concern). And frankly, kobolds are merely a nuisance. As soon as they're spotted, they run away and since the actual entrance to my fort has quite a few dogs pastured, they don't get a chance to actually enter the fort before they're spotted and run away.
For a typical ambush or siege, I'll simply open and close the bridge as needed. Building destroyers get a closed bridge which encourages them to path through the cage corridor. Other creatures generally get an open bridge to get 'em inside the entrance room, then I'll close the bridge to prevent them from continuing to try and path their way back to the uncontrolled area once they decide to flee forcing them to either path through the cage traps, or take their chances with the grinder.