Detection radius, mostly. The upright design allows enemies to pass underneath, while yours forces them around it. This may have the same effect, but pathfinding can be strange at times. Footprint is another issue, yours takes up space and prevents passage, while on mine dwarves can run under it, allowing for quicker civilian escape and quicker chasing down of fleeing enemies. Aesthetics is a smaller issue, mine is an actual watchtower, while yours is more bunker
I think the detection radius would be the same. Don't you lose one space of range for each z-level of difference? I'm pretty sure that's the case for ranged weapons, and I've also observed similar behavior when I break into the top of a very deep cavern and as I move downward the area I can see gets narrower and narrower.
Actually, I think it would have a larger detection footprint. The range from the outside edge of the structure would be the same, but the area it covers would be bigger. So you'd need fewer of them. Or have smaller gaps between the same number.
A bigger footprint can also have advantages in that it channels the attackers into areas that can have traps. With stilt-legged towers, you have no clue where an attacker will path. but with 5x6 bunkers, there is a fairly good chance that an attacker will end up passing right next to it.
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. = empty ground F = Fortification
W = Window b = watchbird
h = hatch ^ = trap
So if the attackers are coming from the east or west on any of the 6 rows the bunker blocks, he'll instead go around the north or south end and hit the traps.
These are also easier and safer to build. You can designate all the fortifications at once, then build one set of temporary up/down stairs, then build the roof all at once. Then tear down the stairs and you are completely done outside and can build the hatch and windows from the inside without having your workers exposed to ambushes. Building towers requires a lot more waiting for bits to get built before you can designate the next bits. And it's a lot of trips exposed on the surface for your workers.
This takes takes 19 stones (including the hatch) vs. 18 for the tower, and 10 windows vs 4 for the tower. The stones don't really matter, and the number of windows only matters of you're short on sand (or wood and magma). In my current fort I have no sand, so more windows means waiting for more caravans. Although I suspect the number of fortifications and/or windows might be able to be reduced for the bunker (are the corner fortifications/windows necessary?).
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That's 19 stones and 6 windows. I think that would give as good of visibility, but would an archer be able to shoot in on the diagonals between the corners of the windows?
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11 stones and 6 windows, but I'm even less sure about this one's safety. Can building destroyers get those windows through the diagonals?
One way or the other, I'm starting to like this design.