Castle Rushen is among the best-preserved medieval castles in Europe, its origins lying in a Norse-period stronghold raised to guard the entrance to the Silverburn River. The keep, the oldest surviving part of the castle, dates to the late twelfth or early thirteenth century, the time of the Kings of Mann and the Isles, and was besieged by Robert the Bruce in the early fourteenth century. The castle was steadily developed over the following centuries: by 1313 the keep had been reinforced with towers, and through the fourteenth century an east tower, gatehouses and curtain wall were added. Beyond its role as a royal residence, Castle Rushen has served over the centuries as a mint, a law court, an administrative centre for the Island's government, and, for more than a hundred years until the late nineteenth century, a prison. It is now run by Manx National Heritage as a museum and educational site at the heart of Castletown.