This is one of my favorite pieces- "Miserere Mei, Deus" by Gregorio Allegri, written in the mid-1600's. It's a setting to Psalm 51, "Have mercy upon me..." and talks about sacrifice and forgiveness of sins. It is sung during Holy Week on Good Friday at the Tenebrae service - in total darkness. It is most famous for three things: The "High C" of the treble soloist in the verses; the fact that (as legend has it) at age 12 or so, Mozart put it to paper, after hearing the Sistine Chapel Choir perform this *very* secret piece of theirs; and of course that its composer was a member of the Sistine Chapel Choir, whose soprano and contralto members were all castrati.
The version we perform today, however, is actually a mixture of two different versions (both written by members of the Sistine Chapel Choir)- one on top of the other! This gives it a very eerie sound, indeed. The "High-C" version, was not brought about until about the 1930's, and so certainly was never sung by a castrato, as the last one to be in the Sistine Chapel Choir (Alessandro Moreschi) had already died in the 1920's.
This is a 1987 video of King's College Choir giving a rendition. You can see the solo treble boy on the lower right of the screen. The men to his left are singing the other 3 solos- alto (not contralto/castrato), tenor, bass- and the 2nd treble part is on the bottom left, but he is often off-screen. Notice how easily the kid hits the high notes. ;)