sentences:  The design of the prison was based on that of a contemporary artillery fort.A ditch deep (to prevent prisoners tunnelling out) was placed inside the wall (originally a wooden stockade fence, replaced with a brick wall in 1805) and guarded by 'silent sentries' who could not be seen by the prisoners.The barracks for the garrison were placed outside and a large guard house (known as the Block House) containing troops and six cannon was placed right at the centre.