Balassagyarmat Prison stands behind the old county hall and is one of the town's stranger architectural sights. Built in the 1840s, it is valued for its rare central-plan design rather than for prettiness.
You normally see it from outside, and that is enough for most travellers. Still, it adds a darker layer to the old administrative quarter: courthouse, county hall, prison, churches, and museums all within a few minutes of one another.