Non-obvious entries -- not necessarily opened by a lamp-that's-really-a-lever or even a book-that's-really-a-lever (I mean come on, I read a lot, people would expect it to be a book that opens it) * -- that get you through twists and turns: left, right, up, down, perhaps even diagonally. Connect to what you wouldn't expect to be connected. Be able to appear unexpectedly and freak out your guests. Share it with trusted family and friends. Connect it to basements and other buildings nearby.
Some of it could be decorated like a Jefferies Tube because come on, I want the world to get closer to having Star Trek stuff. But that's just me.
It'd be a secret, and a cool kind of secret.
A house I've been in has kind of a secret, um, crawlspace. You're not supposed to be able to see that, behind a certain wall, there's room to stand and do stuff. I won't say whose house; I won't even quite accurately describe the secret part. Maybe it's deeper than that. MAYBE IT REALLY IS A SECRET PASSAGE. Maybe I'm teasing you with possible knowledge. Because yes, you're thinking about secret passages now, and realizing that secret passages are awesome.
* Maybe the trigger would be disguised as an electrical outlet. ONLY I'D KNOW TO STICK SOMETHING INTO IT.