The dress is a typical example of medieval costumes of the Historicism. Medieval motifs & forms were combined with corset and crinoline. This special belt served almost as sign of 'Medievality' on stages & fancy dress balls.
And also wedding dresses looked different in the 19th century. Low-necked and richly decorated dresses like this were only accepted for evenings and (fancy dress) balls.
It's deceptive, that dresses with the silhouette of evening dresses from the 1850s-1860s are very popular as wedding dresses today.