I've noticed this in a number of starforts I've visited: Casemates are made of brick, but different stones were placed in the wall to support doorways, as it would be too easy to undermine a locked door whose lock stuck into a brick. These stones, however, always seem to be sandstone, which is easily worked, but crumbly and not what I would use to secure a door! Granite can morph into sandstone over time, but usually only under extreme pressure or heat conditions. Could the dankness of a casemate over the period of a century be considered extreme heat conditions?