Conisbrough Castle is one of the most memorable medieval sites around Doncaster, dominated by a tall 12th-century keep that still looks formidable above the village. The ruins are compact, but the shape of the tower and the surviving walls make a strong impression.
It is a good half-day trip if you want something older than Doncaster's market-town centre. The site is managed for visitors, with interpretation that explains the de Warenne family, castle life, and why this particular keep is unusual in England.