Its walls are a metre thick, its internal diameter barely two and a half metres across, and its domed roof rises to the same modest height. It was built, in short, to contain an explosion rather than to invite one.

The structure dates to the early nineteenth century and formed part of the wider Crookhaven mining complex. Its purpose was to store gunpowder, a routine but carefully managed necessity in any working mine, where blasting powder had to be kept dry, stable, and at a safe remove from the workforce. The design reflects that caution in almost every detail. The north elevation, which faces the seafront, carries a blind pointed door and windows, meaning they are decorative rather than functional, built into the masonry but never opened. This castellated treatment was common in the period, a way of giving industrial or utilitarian buildings a degree of architectural dignity without compromising their integrity. The single real entrance is a porch on the east side, and inside, rectangular wall presses, shallow recesses built into the internal wall, would have held the powder kegs in organised storage away from the damp floor. The thick walls and domed roof were not decorative choices; they were calculated to direct any accidental detonation upward and outward, away from surrounding structures.