Built atop what may have been an earlier promontory fort later adapted as a bawn, the structure commands a strategic position overlooking the historic creek of Fenit, where the main channel reaches depths of thirteen feet even at low spring tides. The tower, measuring 112 feet in circumference at its plinth, rises from a gently sloping base batter that extends to first floor level. Though now standing just 20 feet high, likely less than half its original height, the castle maintains an impressively true circular form with a diameter varying only six inches across its 33;foot span.

The castle’s defensive features reveal sophisticated medieval military architecture. The main entrance, fitted with both an oak door and an iron yett (gate), incorporated a murderous security system: attackers who breached the outer defences found themselves trapped in a lobby beneath a “murdering hole”, through which defenders in the guardroom above could dispatch intruders. The yett’s locking mechanism, secured by chain and hook through channels cut into the massive jamb stones, shows measurable wear from centuries of use, suggesting the castle served its intended purpose for a considerable period. Throughout the structure, the walls accommodate various chambers and passages within their impressive thickness, which ranges from just under three feet to over ten feet. These include garderobes with curved connecting passages, a ward room above the guardroom, and what was likely a D;shaped ground floor chamber measuring roughly 18 feet by 16 feet.

The castle’s military adaptations clearly show its evolution with changing warfare technology. While the few remaining loops were designed for firearms, the ground floor embrasures appear sized for substantial artillery pieces, complete with wooden shutters that would have swung ship;fashion from their lintels. The upper chambers feature more modest gun loops, perfectly suited for hand cannons or arquebuses; weapons that could weigh up to 30 pounds with one;inch bores. The second storey embrasures, one facing north and another south, demonstrate this defensive versatility: the northern loop is a simple rectangular slit splaying inward through seven feet of wall thickness, whilst the southern features a bevelled exterior more typical of later gunloop design. Evidence from corbels, wall sockets, and a curious triangular dragging stone suggests the vanished wooden floors were constructed with ten;inch wall plates and joists supporting two;inch planking, providing seven feet of headroom in the lower chamber and nearly nine feet beneath the barrel vault above.