furthermore Federal law trumps state law.
That's only true when the Constitution authorizes Congress to make that particular Federal law.
If the Fed govt makes a law to, say, require states to support Federal socialized medicine, and a state refuses and takes the Feds to court, the judge must find in favor of the states, since the Federal government has no Constitutional authority to impose a socialized medical program (such as Obamacare), on the states.
The 10th amendment states that, in such a case, the authority to implement a government medical program or insurance program rests with the states, and is forbidden to the Federal government.
The 10th amendment is, needless to say, one of the most-violated parts of the Constitution.