Windsor is a historic town in the Royal Borough of Windsor and Maidenhead in Berkshire, England. It is the site of Windsor Castle, one of the official residences of the British monarch. The town is situated 21.8 miles west of Charing Cross, central London, 5.8 miles southeast of Maidenhead, and 15.8 miles east of the modern county town of Reading. It is immediately south of the River Thames, which forms its boundary with its smaller, ancient twin town of Eton.