Congratulations for keeping Furnaceclans running for a year real-time and 56 years dwarf-time. It's a bit mind-boggling, to be honest, comparing the surface between the undead-infestation at its height to now. Where did all the cursed dust lying everywhere go? Picked up/removed when dwarfs walked through it, or just covered up by fresh snow?
Here's a design I use in my underground forts to spot incoming thieves, ambushers, etc.
O, = wall G grate D door + floor
z=+1
O=O=O
=GD+O=
O=O+++
=GD+O=
O=O=O
z=0
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++++++
++++++
++++++
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It positions a guard animal above 2/3 of the entry tiles for the hallway below. Each animal sits on a Grate giving them visibility below, and renders them immune to attack. For your corner design, you could place an animal above each corner square, and possibly a row of them across the entire path to catch leakers.
A more risky method is to have a squad of macemen/hammermen, in full steel, train for several years (pref. above ground) and once legendary place their barracks right at the edge of the map. Channel out a single tile nearby, and use minecarts to fill it with magma. Legendary mace/hammermen shouldn't have any problems with invaders or their subsequent undead corpses, the local magma pit give haulers a place to dump bodies, and they are, in my experience, much better at spotting enemies.
Better yet! Have their barracks located on a series of retracting bridges (the armor stand being on a pillar of ground in the middle) and fight there as well. Once the fight is done, order your squad off the bridge, and pull a lever, dropping the corpses into a magma filled pit 1z level down. Spikes linked to a lever will take care of any animating corpses.
Of course, while both mace/hammerman guard scenarios allow your fort to be 'open' all the time, the odd titan would probably make the guard animal design much safer.
Looking forward to your next video!