[pjmlp]

Yes, ntdll is the lowest level, but you aren't supposed to use it directly, and if you do, well no one is going to help when a patch Tuesday or something like that breaks the application.

The personality DLLs are the applications entry point with the kernel.

[monocasa]

Most of ntdll.dll is officially sanctioned at this point. It's officially documented, and obviously plays into the backwards compat choices they make.

[pjmlp]

Not really, Windows Internals always refers to the few public ones as "take care when relying on this", very few entries do exist on MSDN or Technet, and those that do exist are mostly tailored for device drivers scenarios.

[ChrisSD]

Sort of. Many NT functions are officially documented. But they're also officially documented as unstable. They probably won't break many of the oldest functions but they reserve the right to do so at some point.