The ineptitude of the military seems to have been greatly exaggerated. Goblins and Orcs siege at the same time.
The civs are put on alert and told not to leave the 'megaburrow' that constitutes the entire safe section of the fortress. Work continues normally, but the hunters and woodworkers obediently enter the fortress through the gate before the armies arrive. My military is manually ordered to move to station behind an ambush wall, and they do. How the crossbowmen decided to handle the recent monkey invasion I keep them off the walls, keeping them behind the melee military instead.
The Grass assassin I mentioned before is tied to a rope outside the front gate. Unfortunately except for their hammergoblin leader, all the goblins are crossbowgobs. The lever is pulled to release the creature, hoping that it gets inside before I get to see if the strengthened chitin holds up very well against steel bolts. It doesn't make it. And it dies in literally a single bolt to the head.
The goblins enter the ambush first, and I order all squads to move to a location outside the gate and they all charge as a unit. The goblin leather is cut easily by the iron greatswords and lances, and the hammergoblin leader and his giant rat mount is dispatched by my militia commander with his adamantine greatsword. The goblin's armor hardly slowing the blade at all.
With the entrance hallway now painted a lovely shade of red, the military charges out of the gate and faces the more difficult opponent. 12 Orc Swordsmen in full iron and mithril armor. These orcs are basically just bigger stronger goblins with access to heavier weapons, significantly more dangerous than a goblin, but not the ungodly unstoppable demons some mods turn them into.
But that dosn't matter at all, and my army of 23 envelop their formation, the combat log is chaotic, and it becomes difficult to tell what was happening with all those creatures hacking at eachother at once. But two orcs flee, the rest become corpses. The highly skilled military of Whitecastle has won the day!
11 goblins 10 orcs and a giant rat fall to my military. My casualties only include the tame Grass assassin. and as far as wounded goes, 1 bone carver that took one bolt in his mad dash to get to safety, and a crossbowman that took some moderate wounds from an orc swordsman.
The only bug I encountered was the ranged military not using their bolts.