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 originally was surrounded by a large water-filled moat 31 metres (102 ft) wide. The curtain wall was up to 5 metres (16 ft) feet thick and defended by five D-shaped towers and a twin-towered gate house.
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