I have yet to be invaded, ambushed, or sieged, it's Spring of the second year, and my population is currently over 50. Will my current defenses suffice until I get a proper militia up and running?
Depends.
You may have an ambush get right into that courtyard area past the gate before they get spotted. And then there are fliers. I just had a flock of buzzards drop into my tower which is still open to the sky. Flew off with some food and created some panic, but no serious harm. One died when my military dwarf arrived to see what was up. Rest flew off lugging bloomberry roasts and such.
Not sure of those pups in the passage are on chains or pastured in 1x1 spots either. You want pets covering the full width (they can see at least one urist to each side, but beyond that gets iffy) of any entry that's open to the outside to prevent thieves and ambushes from sneaking in.
Invasions you get warning and enough time to close the gate (assuming the lever controlling it is put somewhere far away where there is usually an idling dwarf around to yank it.) I usually put my gate controls in the main meeting place deep inside.
I don't see your trading post. That (and free and easy traffic flow) are the only reasons for a corridor that wide (beyond looks!) I usually put my trade depot more on the "edge" of things, with a narrower, more easily defended, entry to my main fort past that. It's protected from outside things, once I close the main gate (assuming it comes in through one), but no where near as much as the main entry to my proper fort.
But that's one style. Some people like a lot of fighting and they build more open. But they also get military ramped up very early so it has a real fighting chance.