Prison was rather brutal at this time--and O. Wilde's own incarceration was no joke. Condemned to two years' “hard labour, hard fare, and hard bed" for his guilty verdict under the Criminal Law Amendment Bill (which criminalized homosexuality in 1885, though it's more famous for raising the age of consent from 13 to 16), Wilde and his fellow prisoners experienced the full extend of Reading Gaol's reputation as a terrible place. For example, his hands were shredded in the course of his punishment, due to separating out old ropes into fibers for caulking ships. He and the other inmates also witnessed an execution, which, of course, inspired "The Ballad of Reading Gaol."
All in all, awful. That said, the Victorian era represented a long, slow turn toward more "humanitarian" treatments. Here's one timeline, which shows how some reforms slowly made prisons different (NOTE: not better. different. See Foucault's "Discipline and Punish" for the distinction there--it's own excellent history of penal reform.) Broadly speaking, the movement geared towards reform vs. punishment...although many at the time would have argued that the breaking labor experienced by Wilde and others was good for the soul!
The 19th C. is a fascinating time, if only for the various reform movements going on, esp. within the disciplinary/hospital systems. Here are a few good sources: Women in Prison