I've been rewatching DS9 again, and despite all my years of viewing and familiarity with the show, I still have a question about the layout of the station. All the airlock doors on the station are identical - the large circular doors that roll open. Everyone entering the station from a ship enters through these roll away doors. Everyone departing the station exits through these rolling doors. Presumably people are entering and exiting ships that are docked at the station. So...why are there airlock doors on the Promenade? Ships don't dock there. Yet, in "Rejoined", Jadzia - standing on what is clearly the upper level of the Promenade - watches as Lenara departs the station through an airlock door on the Promenade. But Lenara can't be entering a ship at that airlock, because there are no docking ports at the level of the station's Promenade. Where was she going, then? She wasn't getting into a lift, because those have different doors. What is going on with people entering and exiting directly to and from the Promenade?
No doubt the internal airlocks are there to serve as barriers between the various sections of the station. There would be the simple prudence of securing against catastrophic hull breach or spread of contaminants - a danger both due to the mining and refining functions, and because the station was there to intimidate Bajor and would face the wrath of Bajoran suicide bombers and possible other enemies of Cardassia. In addition, there would be the need to provide security barriers in contingencies: these airlocks were shown to feature weapons detectors and countermeasures ("Captive Pursuit") and would be effective in slowing down a rampaging boarding party or worker uprising. Finally, there would be a need for security barriers even in routine operations, as we saw Bajoran workers frequent the Promenade area ("Necessary Evil") yet also work in the outer zones and travel by ships docking at the outer ring. Quite possibly, such "airlocks" would exist at a great many locations inside the station for these three purposes; only the habitat ring, dedicated to trustworthy Cardassians, would have lots of regular doorways and few "airlocks". Once the station became a trading hub under joint Bajoran/Starfleet control, the security arrangements would become a nuisance. But it would not be easy to rip them out. Also, just as you point out, there is no direct way to access the Promenade (say, an outer docking port or a transporter), yet the Promenade is the place everybody wants to access, so retaining the security systems would be quite reasonable even for the new overlords. Original set plans for the Promenade did include a multi-pad transporter, but it was never really constructed nor indicated to exist off screen. So other means of channeling the flow of people would have to be used, and hefty rolling gates sound just as good as any... Incidentally, there are episodes where the station is indicated to harbor multiple vessels but the exterior shots show none docked ("Dax" is especially notorious). Perhaps this means that many vessels of smaller types actually dock at the habitat ring, in those bays also used by the runabouts, and for that reason are not visible even when present? This probably was a Cardassian practice already, considering the very existence of those bays. Passengers from such vessels would then be guided to walk to the Promenade along heavily secured routes featuring these rolling gates. Timo Saloniemi
It could be that those airlock corridors have a turbo lift at the end, a turbo lift that travels directly across the transfer bridge (those three spokes connecting the core of the station with first the habitat ring and then the outer docking ring) towards one of the primary docking ports. So the turbo lift would first go down (Promenade is higher than the uppermost deck of the transfer bridges) and then horizontally along to one of the primary docking ports. Or for some reason the corridor (which ends in some tiny kind of T-junction) first splits and then comes back together again, before sloping downwards somewhat (since the ground floor of the promenade is probably at least ten metres above say the highest level of the transfer bridge) until becoming flat and heading straight out (in one big long corridor) directly to the primary docking port. Or maybe it was for plot convenience and the blueprints were thrown out of the airlock...
Of some interest is the mismatch between the sets and the supposed dimensions of the station. In the pilot episode already, the station was essentially "blown up" to at least twice the originally intended size, so that the E-D could neatly fit between the docking pylons. This left the easily recognizable Promenade windows badly out of scale; it also meant that the placement of those fancy airlocks, perhaps originally intended to suggest six of them overall at regular intervals, now had to be treated differently. If we assume airlocks at regular intervals, and ignore the exact curvature of the set (easily done - we've been doing that with the various Enterprise corridors since the sixties!), there'd now be more like 12 of the airlocks, or perhaps 15 or 18. Then again, we can also assume there are closely spaced pairs of them, with gaps in between - possibly one pair per each of the three wide connecting bridges. Timo Saloniemi