Clifford's Tower, York

The keep of the medieval Norman castle is commonly
referred to as Clifford's Tower.  Built originally on the
orders of William I to dominate the former Viking city
of York, the castle suffered a tumultuous early history
before developing into a major fortification with
extensive water defences.  After a major explosion
in 1684 rendered the remaining military defences
uninhabitable, York Castle continued to be
used as a jail and prison until 1929.

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English Heritage
York Castle