What is the main conflict in Sharpe's Siege: Richard Sharpe and the Winter Campaign, 1814 by Bernard Cornwell?

Major Richard Sharpe takes command of three companies of men during a diversionary attack on a French fortress and environs. Under Sharpe's resolute leadership the men fight a series of engagements with decisive results until falling back on a largely-destroyed fortress where they discover they have been abandoned. They defend the fortress during a bloody but abbreviated siege that ends in an unusual surrender.