As you probably know, there are some relevant things here:
Also a lot of silliness. Hopefully this thread can stay a bit more serious.
If you start on a foothill, use the terrain to your advantage - fences are nice defenses for narrow ledges.
Mountains are highly defensible, but hard to live in - for now, found your settlement farther down.
In the plains, lakes are a nice substitute for moats or trenches, though those can help too.