object/nightwatch/12 matches the site

public static HumanMadeObject Nightwatch_12_Materials()
{
    var nightWatch = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/nightwatch/12")
        .WithLabel("Night Watch by Rembrandt")
        .WithClassifiedAs(Aat.Painting, Aat.TypeOfWork)
        .WithMadeOf("oil", "300015050")
        .WithMadeOf("canvas", "300014078");

    return nightWatch;
}

• Metatype classification The second argument classifies the first, not the object: WithClassifiedAs(Aat.Description, Aat.Statement) says this is a Description, and Description is a kind of Statement. The Plain C# form shows the nesting.

var humanMadeObject = new HumanMadeObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/object/nightwatch/12",
    Label = "Night Watch by Rembrandt",
    MadeOf = [Aat.Oil, Aat.Canvas],
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300033618", "Painting")
        {
            ClassifiedAs = [Aat.TypeOfWork],
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.