The castle was subsequently completely rebuilt; earlier historians had concluded that the rebuilding occurred in the late 13th century, but archaeological excavations in the 1970s demonstrated that the work occurred between 1225–50. The new castle had a concentric pentagonal plan, with thick, embanked walls. The castle had a gatehouse, although later damage means that its design remains uncertain. Historian John Kenyon concludes that Banbury Castle is "remarkable for its early concentric shape", which is usually seen in somewhat later castles such as Harlech or Beaumaris. By the second half of the 13th century the castle was being used as a prison by the bishops of Lincoln.