Durham Castle faces the cathedral across Palace Green, forming part of the UNESCO World Heritage Site. It is a working university building — students live and eat here — which gives guided tours an unusual intimacy: you see the medieval keep, the 14th-century great hall, the Norman chapel, and the impressively carved Black Staircase alongside students' bicycles and notice boards. Tours run daily and are the only way to see inside.
The Norman chapel is one of the oldest parts of the castle (1072) and retains original carved stonework, including a remarkable capital depicting a mermaid. The Great Hall, remodelled in the 14th century, is still used for formal dinners by the college. Palace Green itself — the wide grassed area between castle and cathedral — is one of the finest civic spaces in England.