Well, I dumped my attempt to constructively renovate this fortress and tried just opening the gate as someone above did. I put all of the military right behind the bridge. All but two of those were killed. The marvelous commander was of course one of the survivors. The seige was broken with relatively few civilian lives lost (which part of the word lockdown did they not understand?). The ensuing tantrum spiral, however, took half or more of the civilians. I got a Dwarven bone bin out of that, but lost the bridge. A small wave of migrants arrived, followed by the Elven caravan. Goblin ambushes started popping up then, and I quit. The only way I can see to improve life in that fortress is to lock the gate and spend a lot of time digging.
My comments on the fortress design:
IMO the barracks ought to be between the gate and the main stairwell, not behind the main stairwell.
If that is an archer's ledge above the bridge, it ought to be accessible from the Dwarven side rather than from the enemy side.
The use of the minecart to make a single humongous quantum stockpile was interesting, but makes it hard to find things at first.
Dumping all of the refuse in one pile, down a shaft, does contain the miasma but also removes potential moody Dwarf materials from circulation.
I felt cramped in this fortress, and I didn't understand the odd digging to nowhere off in the corners of the map. I missed having magma workshops.
That military commander was amazing, though.