19 My very first base in the very first world I started in Vanilla was dug right into the side of a cliff. I liked not having to worry much about zombies standing outside my door, even though I switched into Peaceful mode after a skelly dropped some bones on my doorstep the first morning.
Every Survival base I've built since then has been based on that, although I've never been able/motivated enough to find nearly as nice a cliff since then.