Oh man, I didn't even realize you were using Dig Deeper, QUICK build a hallway with upright spikes attached to a lever!
This. I enjoyed dig deeper but it was always a mad dash to get SOME kind of defense set up within the first season. I remember once I had all 7 of my starter dwarves huddled in a room with crossbows trained on the door. They killed all the orcs, but only 3 walked out of that room. Was good times.
Oh and from having done a similar set up in my early days to what you're proposing, I can tell you it's bad news bears. And that was
without Dig Deeper.
I generally do my set up like so (side view):
..d---------------d....
--u..........B.........u---
Where d and u are down and up staircases, B is my barracks, and the dashes are solid wall, dots are corridors. This forces my attackers to plow through my defenders before they can even get to the main fortress. If my entire army falls, well everyone else is screwed anyways.
Also I generally have a large cavern before that where I set up traps and carve fortifications into the wall. In 40d, it was a large shooting gallery. In 31, it's just a big room, so I've stopped doing that set up for the time being.
You should put floodgates across your entrance, and link them to a lever far back in your fortress proper. You can leave them down until attackers show up. When you have seigers, then you pull the lever, and you can defend as wide a hallway or entrance as you like.
For the record this doesn't work in the 31 version, if that's the one you're using. Goblins come with Cave Crawlers and Trolls now, which are both building destroyers.