Honestly, the wiki article on Hinzelmann's myth is accurate to what is shown in the novel.
There's this part from the book about his true form...
Where Hinzelmann had been standing stood a male child, no more than five years old. His hair was dark brown, and long. He was perfectly naked, save for a worn leather band around his neck. He was pierced with two swords, one of them going through his chest, the other entering at his shoulder, with the point coming out beneath the rib-cage. Blood flowed through the wounds without stopping and ran down the child’s body to pool and puddle on the floor. The swords looked unimaginably old.
The little boy stared up at Shadow with eyes that held only pain.
American Gods, Chapter 20
... which is also referenced in the article...
In one tale he showed his true form to a maid, who fainted; it was that of a small child, around four years of age, stabbed and slashed with two swords.
The myth mentioning a castle is only a part of his history - Wikipedia states he started haunting Hudemühlen, in northern Germany, after he was banished from the forests of Bohemia. This appears to be where Gaiman tinkered with his origins slightly - Hinzelmann mentions the Black Forest (which is a small range in southwest Germany) during his reveal rather than Bohemia (which is basically the western half of Czech Republic). There's several possibilities that could cover that discrepancy - intentional background change by the author, this American version of Hinzelmann being based on a mangled misremembered myth, or Hinzelmann's tribe actually creating him in the Black Forest long before our mythic history finds him in Bohemia.