From the guidebook:

Feudal custom limited the extent and strength of private fortification and by the 12th century, crenellation (battlements), distinguished a castle from a manor house.  Any castle built without permission was known as an adulterine and could be forfeit to the King.  Sir Richard, the first Lord Scrope, a loyal and distinguished knight, was granted his licence to crenelate in 1379.  Bolton Castle was built on the site of the original manor house and, at an annual cost of 1,000 marks, was completed in 1399, for a total expenditure of £12.000 – an incredible sum in those days.

Although by its very nature, Bolton Castle was built for defence, it was also an extravagantly luxurious dwelling.

On the north wall are the motto, Fiel Pero Desdichado (‘Faithful but Unfortunate’) – a reference to the loyalty of the 1st Duke’s father to the Royalist cause after the execution of Charles 1 in 1649. Four terracotta plaques on the walls represent rivers that played a part in Marlborough’s campaigns during the War of the Spanish Succession (1702-13): the Danube, the Rhine (with the sirens of the Lorelei), the Meuse and the Moselle. The terracotta medallions at the corners represent the 1st Duke; his wife Sarah; Queen Anne; and Prince Eugene of Savoy, Marlborough’s most steadfast Continental ally, who played an important part in the victories of Blenheim (1704), Ramilles (1706), Oudenarde (1708) and Malplaquet (1709). The low-relief frieze around the ceiling evokes the Marlborough wars. One panel shows the defence of the village of Blenheim, the climactic moment of the 1st Duke’s most famous victory.  *

*From the National Trust guide book