Truce Zone: About as rare as the Neutral Zone, its counterpart trope, in the large, but mention should be given to both the Free Eilish Confederacy on the one hand, which occupies the majority of the Eilish Expanse constellation and whose policies of firm neutrality and not-terribly-restrictive legal system make it both a place where almost everybody is welcome and one of the favorite places for galactic intelligence agencies to play their away games (like, as it were, Casablanca IN SPACE), and on the other, to the red market which occupies most of the approximately-habitable planet Nepscia (Galith Waste), which welcomes anyone with money and guns.
(Well, technically it welcomes people with money and no guns even more, but they rarely find the welcome enjoyable. Shortly after which they rarely find anything, anything, any more.)
City of Spies: Many spies, of course, hang around Nepscia (Galith Waste) and its red market, both because many secrets find their way there, and because a city with no rules (see: Wretched Hive) makes an excellent place to play intelligence agencies’ rougher away games. Likewise, the Conclave Drift contains a lot of spies, simply because it’s where everyone is, and where a lot of top-level diplomacy and politicking goes on, with the obvious concomitant to it.
Nonetheless, the true City of Spies remains Eilan (Eilish Expanse), the capital world of the Free Eilish Confederacy, a doggedly neutral power friendly to the world, conveniently central for most of the Great Powers while not being – unlike the Conclave Drift – too convenient for any one of them in particular, and as such, the absolute favorite location for people’s intelligence away games. Absolutely crawling with agents for absolutely everyone. They don’t quite have carpooling for the tails, but it’s certainly not unheard of for two tails on the same agent to end up having to shamefacedly exchange name, address, and insurance information after a flitter collision…
Capital City: Of the Associated Worlds as a whole, that would be the Conclave Drift, the giant habitat in which the Conclave of Galactic Polities is situated. It’s also a major commercial and cultural hub, it being – due to the room it sets aside for every polity in the Accord to build its own little mini-city – one of the few places you can find everyone together, and its markets are one of the few places in the Worlds where you can rely on finding just about everything that it’s legal to buy anywhere. (The Empire had this in mind, of course, when they donated one of their star systems to build the thing in.) It is slightly subverted inasmuch as not all that much freight gets transshipped through it; it would be rather out of the way. But a surprisingly high percentage of the actual deals get made here.
Other candidates for major commercial centers would be Mer Covales, on Seranth (Imperial Core), which houses several major commercial exchanges and which does play host to a great deal of manufacturing and transshipping; and the worlds of the Free Eilish Confederacy, whose policies of neutrality and openness make it a favorite spot for business – and also a favorite spot for back-door politics and for galactic intelligence agencies to host their away games. Neither of those, of course, are political capitals of anything.
Of the Empire, that would be Calmirie (“center of order”), which is both the political capital of the Empire, and a significant commercial (somewhat overtaken by Mer Covales) and cultural (somewhat overtaken by Delphys (Imperial Core)) center. It plays it essentially straight.