Crowley's Monster Prison was a prison used by Crowley to capture and interrogate monsters and Alphas.
In order to locate the doorway to Purgatory, Crowley sent out demons and hunters to capture various monsters and bring them to this prison for interrogation. Lesser monsters were interrogated for the location of their respective Alphas while the Alphas themselves were interrogated for the location of Purgatory itself.
Crowley employed members of the Campbell Family, along with Sam Winchester, and later Dean Winchester as well. Crowley gained their help by promising the return of Mary Winchester for the former and Sam's soul for the latter. Castiel, who also sought out Purgatory for its souls, worked with Crowley on capturing monsters.
The prison was eventually raided by Sam, Dean, Castiel and Meg. The four managed to "defeat" Crowley. However, Crowley had faked his death with Castiel's help and resumed capturing monsters without Sam and Dean knowing.
The prison and a majority of its inhabitants were killed when Castiel destroyed the prison. The sole known survivor was the Alpha Vampire.