There's an infinite, fractal, size and complexity to the city, while being constrained on all sides. You always know where you are, but there's a near infinity of places to be. It's both surprisingly old and cutting edge new, and it generates its own history. It's the only city I know of that looks exactly like the photos of it.
You can set out at one river, on a journey across the island, and it feels like a vast quest among the roots of a giant alien steel, stone and glass forest (with massive iron snakes coursing beneath the earth), and before you know it, you're at the other side, looking at the view of somewhere else across a river -- yet entire generations of lives have lived, loved and died without ever seeing the outside of it.
In that short walk you'll cross betwixt a population greater than most towns, encounter nearly every method man has devised to cross the earth, meet poor artists and giants of industry. It can be stunningly hot, or buried in snow and freezing depending on the weather. Rain storms can start and stop in the time it takes to read this sentence. In a few moments, you can be beneath the surface of the earth, or higher than the first aircraft dared to go, without ever taking off your jacket or adjusting your shoes.
Every building you step into captures a moment in history of good ideas and bad, a museum of thought, a classroom of fads and fashion, structure and freedom. It's like a random sample of the whole world was picked up and air dropped onto a carefully organized vessel, a Rama waiting to lift up and go forth on its journey elsewhere.