Is there a prison in Santianmen?

Yes

Relevant information shows that there is Huzhou Prison in Huzhou City in Santianmen.

Huzhou Prison is located in Huzhou Economic and Technological Development Zone, Zhejiang Province. It was formerly known as Huzhou Labor Reform and Discipline Detachment, and was renamed Huzhou Prison on 1994. Address: Santianmen, Huzhou.