Your setup is similar but suffers two major drawbacks which I've modded out of my design. 1) Fortifications won't stop arrows. Any elite bowman will ignore them completely and slaughter your poults, and 2) The hatch can be walked to, so a building destroyer will happily walk over and smash the hatch open. I solved both of these issues by using windows instead, and flooring over the whole thing. It's harder to reload, but you never NEED to reload, so it doesn't matter.
I've got poults coming out of my ears. Out of
their ears. They breed like rabbits[1].
So, anyway, they're expendable. I don't care if they get shot. I'd rather lock them in and keep them where they are, at least until they become adult and prove to be either the best breeding stock or suitable only for the butchers. (Must build more butchers workshops!) Then I'd swap them out with other
sacrificialattentive watchbirds.
Plus, if a BD can walk to the hatch, they're well past the defences that this pillbox is, given the only (non-flying) way to it is to get through the defence-orientated entrance-way, etc.
(And flyers who get there are at least kept busy, I suppose.) The pillbox is also future-proofed against (non-flying) Construction Destroyer activities, of course, but that adds little current value to the structure.
As far as external snipers, I did say that windows would help as well, indeed, and replacing the down-stair above with a floor/wall or even putting walls up on the access catwalks would help with the BD issue, of course, but both would make it not so easy to drop a "belly-gunner" bowdwarf into there to pick off an otherwise awkwardly-located enemy...
About the only issue I might have is line-of-sight range restricted to that given by the given livestock, but the eventual fort design will have highly constrained paths leading them right past various pillboxes well in advance of causing any danger....
(Hmmm, wonder if I should upload my map to the map-upload thingy. It's only five years in, and very much in its infancy. Never done it before, but s a kiloproject (not quite a megaproject...yet), there are several other aspects about the fort design that I wouldn't mind sharing.)
[1] As do the rabbits, as I've also been collecting every single species I can. I had to slaughter the trapped-and-trained elephants because they slid over into starvation, but other than that I'm rotating the pastures[2] of and selectively culling primarily the grazers, at the moment[3]. Back to the birds, however, I had a similar population 'egg'splosion issue with a previous fort, and laid out an array of unconstrained 1x1 pastures out in the wilds to give them room to breath and which was also good for detecting ambushers, although caused issues with dwarfs wanting to go out and get the poultry back in their pasture when they fled in fright. i.e. sending my little innocent animal-hauling workers potentially right into the fray... also, choosing guard-poultry with "g" as their map symbol (i.e. guineafowl) confuzzled things mightily, 'cos they look very much like the goblins they're in the middle of...
[2] I have (I think) 30 separate 10x10 pastures at ground level within my compound (roughly 100x100, walled in and ditched on the outer, and there's space in-between them all as they emulate the subterranean corridor/room layout), with room for a few more, and two layers of soil below only partly employed for other purposes so giving me 100 total pastures of that size... Currently the large (and mostly feline) carnivores have a pasture to themselves (as do the top-notch selection of male birds of all species, and I've just made a new hatchery for along those lines), and haven't yet gotten annoyed with each other, but I'm prepared to split them up if they do start complaining before I get cubs of the relevant species...
[3] And eventually I'll use eugenics on any creature not subject to grazing limitations,