An entire elevated corridor, almost a kilometre long, built by Giorgio Vasari (longtime and mostly deserving villain of Zak's) to join together two Medici properties that were, among other things, on two different sides of the river. The commitment of the Medicis to their creepiness and to leaving a decent dungeoneering legacy is really jaw-dropping, especially considering that this is a passage that passes through a priceless and highly heistable collection of paintings, crawls laboriously around the tower of a rival family that wouldn't let them take a shortcut, dances over the shops lining the bridge (the Medicis had the butchers kicked out so they wouldn't stink up the passage, and replaced them with goldsmith,s who are again highly heistable: centuries later Mussolini installed windows so Hitler could enjoy the view), passes through a church so the family could observe Mass privately (and so your party can shoot poison darts at a bishop), branches off into a side entrance in a grotto in some artificial gardens and then finally ends inside one of the world's ugliest and most villainous buildings, the Palazzo Pitti, which, I regret to inform you, looks like this:
|This is why people who think the Medicis are responsible for the very|
concept of beauty are wrong and bad.
I really can't think of any reason you wouldn't drop this into one of your cities. It's not alone, either: different power-centres can have different ones. The Pope has one in Rome that sadly doesn't cross a river and doesn't intersect with many other buildings but is still quite impressive:
Particularly given that two of the buildings it does intersect with are St Peter's and the mausoleum of the Emperor Hadrian, subsequently extensively retrofitted into a Papal castle.
Quite aise from the neverending fun of a good corridor fight, these things are like portable wormhole carriers in a more scifi game: they warp people around the city in exciting and risky ways. You could have a city of intrigue and decay where passages like this slink over and under one another and guards patrol them endlessly trying to ensure nobody is drilling from into another; or one where they really are (quasi-)wormholes, sorcerously maintained and liable to implode at any moment; or one where they pass through spaces nobody has accessed for centuries, sudden chasms and walled-up side passages like the Paths of the Dead. True, deep-level dungeoneering without ever having to leave the Duke's palace...