There are dozens of ways to build a world, but my own personal favorite involves a starting point of one or two very simple rules from which everything else can, and must be, derived. I followed this approach with
The Inferior, but there are lots of more famous examples out there.
One of the most amazing feats of worldbuilding ever achieved, in my opinion, is Brian Aldiss'
Helliconia. I've spoken about this place before: its teeming, crazy biology; its religions and philosophies; its continents and seas. Two thousand pages of stunning creation.
But what is most amazing to me about the place, is that the entire thing may have been derived from the following "law":
"The seasons last for centuries."
Everything follows from there -- the biology *must* be adaptable to an 800 year winter. The orbit of the planet *must* be designed in such a way that these lengthy seasons are physically possible. Human history and religion and thought and warfare cannot escape the influence of climate and so on.
Another, more recent example of this approach to worldbuilding, is Chris Wooding's
The Fade. The genetic code of his world might go something like this:
"A planet full of caverns where the surface is uninhabitable."
Once again, marvelous complexity springs from a very simple premise and none of this is hurt by good characters and a great adventure storyline.
There are pitfalls to this approach too, of course. One of the most obvious is the fact that in real life, not all lines are straight and nature revels in experimentation and "mistakes". There must be nuance: some of your shades of grey should have shades of grey in them...
What are your favorite examples of rule-based worldbuilding? Does the concept even make sense?