When it was built in 1915, the Alcatraz cellblock was the largest concrete structure ever built. At that time, Alcatraz was a military prison. It wasn't until the 1930s that the federal bureau of corrections took over the facility and turned it into a high-security federal prison.
Alcatraz is a very small prison - there were never more than about 250 prisoners there at one time. There's only one cellblock, and the capacity was about 350 prisoners.
I found the cellblock to have a wonderfully light and beautiful architecture. The place must have been bitter cold in the winter, though.