The build is pretty simple; you can adjust to use whatever you have available and probably make it look better.
1) Start by making the base. I prefer to put 1x1 round plates in this pattern, to match the 1-inch marks on printed dungeon times or battlemaps, but you can also make them with 4x4 plates if you want more space for the minis and don't care about compatibility with the standard grid. Add a technic peg in the back:
2) Add height. This one is a rough cave-like wall, but you can also go with a masonry wall and have the top bit be a balcony:
3: After making the wall nine bricks high (eight on top of the base pin), add the ledge or balcony. This is a good use for those plate-width bases, but if you don't have those you can stack plates together. Put another Technic pin on top:
Another fun option, although much less robust, is the little eroded crag island from the Tiger Widow Island set:
I downloaded the instructions and mass-produced them, because it was a good way to use up that big drill piece, or larger engine pieces that otherwise have no place in a fantasy world. I ended up using them a lot to represent floating islands in my Spelljammer game:
If you were not running a sea or space game, you can replace the blue base with gray and have them be mountain terrain.