Most of the castle is built of Spilsby greenstone, as are several nearby churches. The local greenstone is a limestone that proved to be porous, prone to rapid deterioration when exposed to weather and a substandard building material. The castle was constructed as an irregular polygonal enclosure. The castle is one of the earliest examples of a uniform castle designed and built without a keep. It was originally surrounded by a large water-filled moat 31 metres wide. The curtain wall was up to 5 metres feet thick and defended by five D-shaped towers and a twin-towered gate house.
Similar to another castle built by Ranulf during the same period at Beeston in Cheshire, Bolingroke had no inner defensive keep. The castle relied instead on thick walls and the five D shaped defensive corner towers. Some design similarities are noted with the contemporary castle at Boulogne-sur-Mer, France that was also constructed without a central donjon.
Copyright: CC 3.0