Editing Bolingbroke Castle (section)

==Construction==
[[Image:Boly towers.jpg|left|thumb|200px|Reconstruction of the north face of the castle, showing the gatehouse]]
Most of the castle is built of [[Spilsby]] [[greensand (geology)|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]] {{convert|31|m}} wide. The [[Curtain wall (fortification)|curtain wall]] was up to {{convert|5|m}} 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 Castle|Beeston]] in Cheshire, Bolingbroke 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 [[Château de Boulogne-sur-Mer|Boulogne-sur-Mer]], France that was also constructed without a [[keep|central donjon]].