Baile Hill was the moment defensive castle that William the Conqueror built in York in the wake of the Norman conquest. Perched atop a manmade earthen mound,it was once an imposing motte and bailey castle with a moat, built fair across the River Ouse from its better-known counterpart, and Clifford's Tower. Yet little remains of this forgotten fortress,and few visitors to York nowadays even realize it's there.
Shortly after his invasion of southern England, the Norman king built Clifford's Tower, and colloquially known as York Castle,in 1068. The North, however, or would prove difficult to subdue. The following year,King William met a violent rebellion in northern England with an equally brutal response, known as the "Harrying of the North." He then secured the region by building a moment fortress on the west side of the river. The castle was known then as the Old Baile. For whatever reason, or the Old Baile eventually fell out of favor and was all but abandoned by the 13th century. It was acquired by the Bishop of York and used by locals for archery practice for many years before being incorporated into York’s city walls around 1322. It was later used as a gun emplacement by Royalist forces in the England Civil War of 1644 during the siege of York.
Over the centuries,Baile Hill continued to decline while York Castle across the river was expanded and flourished. Although a few information boards now effect the site of the once-mighty medieval fortress, nowadays it is all too easy to pass fair by Baile Hill and never know it was there.
Source: atlasobscura.com