Troika is the City in the Middle of the Road, not of Windowless Rooms, nor of Doors. These are different places that one can get to through different means. Troika is reached by those who travel and have no destination. When purpose is abandoned its thick metal walls might loom on the horizon or peek above the trees to guide you to its open gates.
Its gates open on to the million spheres each with a road meandering away to be traveled by citizens with purpose. They step out with a destination in mind and travel until the city is out of sight, arriving where they intend to be and the way back wholly obscured.
To get back to the city is easier than first finding it; walk a road with nothing behind you and nothing ahead. Leave no one behind wishing your return, walk with no purpose, and Troika will open its gates to you again.
Bureau of Universal Exchange
Scattered throughout Troika are the silo-like Bureaus of Universal Exchange. Windowless and towering —either occupied because of these facts or floors and shutters have been welded on— one enters the ground floor to deal with its spider-like proprietors.
You find them hunched beyond tiny portholes and pass through your goods and currencies to be fondled by their hairy little hands. They have an uncanny head for the value of things, they will never confer, you will never get a bargain, they will instantly offer only exactly what it's worth and not a penny more, minus a small handling fee of course. Roll 2d6 to determine the % they add on top that day. Reroll 6s and keep adding them until you stop.
They will exchange anything for the Troikian penny, a brass coin of muddy source. The currency is worth whatever the standard currency of your game is, but only of face value to those who can travel to the city, which, while a large number, is far less than the entirety of the population of the universe.
When bringing them alien currency randomly determine how valuable it is at that exact moment. Roll 2d6 in order. The first is the penny, the second is foreign currency, thus determining the ratio. Always add one in the bureau's favour. They're running a business after all.
If a currency is a step above the base (gold>silver for instance) then just add a zero in its favour.
You can list these for later use if you like, but it's even easier to determine it anew each time. Time and space is unpredictable, change is relative.