I do this all the time. It's pretty easy. All you need is a door, and preferably an elevated location.
Set up the fortress so that you can restrict access to a single door. Be careful that you don't allow a thief to take an alternate locked doorway and open up a 2nd entrance. On the approach to the one door, build a long corridor, maybe 30 tiles long. I have mine 3 wide as it leads to the trade depot. Set up your dwarf so that it can fire down the length of the corridor and have the doorway maybe 5 tiles ahead of him, on the side of the corridor.
When the orc siege arrives, restrict passage to only that door. As they reach the corridor, it'll become apparent that they are pathing to the doorway and your dwarf will open fire on them. Before any of them reach the door, lock it and they'll all continue until they bump into the locked door and then they'll stand there with no new place to go and get cut down. Once they're all dead, reopen the door for the next group to path in. Repeat until the siege is broken.
If your dwarf is a z-level above, he will rarely if ever get shot at. At the ground level you run the risk of some snipers pathing to the fortification and shooting in, but I've never seen orcs try to shoot through fortifications unless they're just a handful of tiles away.
The downsides to trying this are that you run a decent chance of having caravans wiped out, but that's somewhat inevitable no matter what you do. Also if you get war elephant mounts, then the door just won't cut it. I've yet to gen a world with those, so I can't give a strategy there.
In one other no-immigrant fortress I have a single dwarf with over 1000 kills. She's been the only military the fortress has ever had with 15 years of gobbo ambushes and orc sieges. No traps, though a few got smashed by bridges, but not many.