Built in the early 1100s, Sleaford Castle was originally built as a manor house rather than a defensive castle. In 1216, King John stayed there after his crossing of the Fens in which he infamously lost a lot of his possessions. All that stands of the castle today is part of a wall in the far end of the field, reported to be part of the inner bailey. There are also numerous mounds showing the foundations of the castle.