http://mkv25.net/dfma/map-4421-pillarsnarling-sum-204Entrance is only half finished, the fortress is about a third done overall.
I might restart from my earliest save for this spot knowing what I do know about the geography.
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My primary purpose for the odd pattern at the ground level entrance is to force the pathfinding routine to always path the shorter route where I can station my defenders so they will be safe from arrow fire and still allow a caravan to approach. I find the pattern does not cause traffic problems and makes for a nice choke point.
Once the entrance is done, raising the draw bridges allows the dwarfs to continue the use of the dump above the entrance. Below the entrance is the sparing and archery areas which when completed gobos would have to path below the the trade depo and through the sparing area in times of distress, at the moment there are no soldiers as I don't have the population for it.
The rows of catapults facing nothing are part of my "training and reeducation" program, by the time my dwarfs have completed their regime of training they usually have nearly maxed status(busy dwarfs party less...). The purpose of the intensive training is to ready my fortress for a huge population influx by training 30 or 40 dwarfs really well in the necessities and everyone else gets to be commoners/soldiery/fun killing time.