Castrati (castrated male singers) were not caused by opera. They were the result of the church’s prohibition of female singers in church. The English went with boy sopranos and falsettists to deal with this issue, but in Italy they settled on this artificial source for sopranos and altos. A number of castrati congregated in Rome where they performed the austere counterpoint of Palestrina with lots of added ornaments. It is my understanding that they all extemporized ornaments simultaneously in all voices, at least until it was prohibited. This is a little hard to imagine, but might be like five Mariah Careys singing in 5 part counterpoint.
This was the state of things in Rome when Virgilio Mazzocchi (1597-1646), Michel Angelo Rossi, Marco Marazzoli (d.1662), and Luigi Rossi (1598-1653) began writing operas there in the 1620’s. While Monteverdi and the camerata had composed for natural female sopranos and male tenors, in Rome in the third and fourth decades everything was sung by men, including many castrati—all ornamenting away on the monodic lines. [For a discussion of monody see here.]
This became the model for Baroque Italian opera everywhere. In Venice and Naples females appeared on stage with the castrati, but the male singers were the stars, and the women followed their lead. We may infer how they sang from what we know. It was a technique designed for extemporized ornamenting, probably never exceeding a medium amount of weight in the voice. The weight will come from the male voice itself and not from an imposed heavy sound such as we would hear in Verdi today. I will assume that the term leggiero, between legato and staccato, applies to the style of this period.
Almost a century later, Handel went to Rome to observe the phenomenon at first hand, and to try his hand at composing for the great male singers at Rome. We know from Opera Proibita that this was some of Handel's most spectacular vocal music.
We may infer from what we know that a castrato mezzo-soprano conveyed a sufficiently heroic sound to be convincing as the dramatic hero. I’m going to guess that Marilyn Horne in Vivaldi’s Orlando Furioso is as close as we get to this sound. We may not believe in how she looks, but we believe in her sound.
By comparison a tenor singing with the same technique was not at all interesting to them. Tenors could not compete with the extreme virtuosity of the treble castrati.
Eventually other opera came along that was not Italian. The French thought castrati were disgusting and composed only for natural voices. French Baroque opera means Jean Baptiste Lully (1632-87) and Jean-Philippe Rameau (1683-1764). French Baroque opera with its strong ballet component has never been wildly popular in my lifetime. The French loved Rameau’s Platée, of all things. We know that Platée includes a coloratura aria for female soprano, but everything else could be sung by anyone. Technically everything seems pop song light.
The late Baroque also includes a professional German company in Hamburg between 1696 and 1734 which featured operas by Reinhard Keiser (1674-1739). We can’t say much about them technically. Isabel Bayrakdarian sang Cleopatra (1704) by Johann Matthewson (1681-1764) which was written for the Hamburg Opera. That may be my first experience of this music.
The late Baroque featured an intense rhythmic drive that stirred excitement. In the 21st century we are most familiar with this in the works of Handel and Bach, both Germans.