Maybe you were disappointed to learn your immigrant ancestors were among the huddled masses who entered New York Harbor before Ellis Island opened in 1892—which means they're not in the massive, well-known passenger-list database at www.ellisisland.org. That online mecca basks in a much-deserved limelight, but it also overshadows Ellis Island's humble precursor: Castle Garden.

Between 1855 and 1890, the landing depot there received more than 8 million immigrants, primarily Protestants and Roman Catholics from Western and Northern Europe. And though Castle Garden resources aren't as easily accessible as tools for that other doorway into the United States, you can follow these four steps to shed light on ancestors who disembarked there.