So . . . purely hypothetically . . . . let's say you built a barn building on some land you owned.

[Stinky The Clown]

7. Building into the slope is very easy

I can't imagine why someone told you the land has to be level. This is a picture of an old barn that is at ground level on the basement and main floor. Barns have been built like this forever. In fact, it is the preferred way to build them as it doubles the "ground floor" space.