Clue: Castle of Otranto's walls besieged by kings (4)
Answer: ROOK
Explanation: Definition: Castle
An envelope ('besieged by') of OO ('**O**trant**O**'s walls') in R and K ('kings'), for the chess piece. *T**he Castle of Otranto* by Horace Walpole is generally regarded as the first Gothic novel, although the clue is perhaps a little closer to the history of the real Castle of Otranto, in the heel of Italy, than the book's plot.