Quoting myself feels weird, but I'll do it:
For example, I've created a vampiric marksdwarf squad, to be eternally stationed on the walls - the Ancient Granite, a guard that never sleeps, eats or drinks, renowned throughout the dwarven mountainhomes and human towns etc. etc. long-winded in character explanation of things
I've personally found it best to make a batch of vampires, outfit them with the best armour available, then kill off their families one by one pre-training to avoid Urist McBadass killing all my other vampires. If he tantrums, he can be put down with little fuss. In any case, because I have quite a few, the loss of an individual isn't that bad. Plus, it's sealed, so they can't kill any civvies.
As to the design, I've never been one for efficiency. I have a sort of courtyard, where the walls are essentially this for three z-levels and several :
W
D
F
W = Wall
D = Marksdorf
F = Fortification
It's facing inwards (the wall is the bit exposed to the elements), which you might think is a bit stupid, but these days (i.e. long after the death of that fort) I employ what I think of as a dual portcullis design. Essentially, the entrance to the courtyard and mountainhome itself is made of vertical bars, which slam down when all the seigers are inside. The people on the mountainhome side are slaughtered by men with pointy sticks/spear minecart shotguns at the end of the passageway. Anyway, the interesting property of bars is that you can shoot through them like there's nothing there. Remember the minecart shotguns? Alternatively (i.e. when I'm to lazy too reload them) I send marksdorfs to each portcullis (secret entrances are great) and essentially create a killing field.
Sorry, I went off on a tangent there...