It is obvious that castration prevents the necessary flow of hormones to stop a boy from reaching puberty, hence the vocal cords cannot enlarge and cause his voice to become lower. As a result, castrati had the high voice of a boy soprano but the lung power of a grown man. Castrati were treated like superstars in the 18th century due to the popularity of opera. They made a lot of money and had throngs of fans. Girls would even seduce them and have sex with them because they were sure they wouldn't get pregnant.
Their training regimen seems to be harder than present-day singers. Wikipedia recounts "The regime of one singing school in Rome (c. 1700) consisted of one hour of singing difficult and awkward pieces, one hour practising trills, one hour practising ornamented passaggi, one hour of singing exercises in their teacher's presence and in front of a mirror so as to avoid unnecessary movement of the body or facial grimaces, and one hour of literary study; all this, moreover, before lunch. After, half-an-hour would be devoted to musical theory, another to writing counterpoint, an hour copying down the same from dictation, and another hour of literary study. During the remainder of the day, the young castrati had to find time to practice their harpsichord playing, and to compose vocal music, either sacred or secular depending on their inclination. This demanding schedule meant that, if sufficiently talented, they were able to make a debut in their mid-teens with a perfect technique and a voice of a flexibility and power no woman or ordinary male singer could match."
Wondering how a castrati's voice sounds like? Listen to the clip below. Comments to that video seem to indicate that this is Alessandro beyond his prime, so his voice quality has already declined. He does not seem to be the best castrato either, but he does hold the distinction of being the only true castrato singer who was ever recorded.
