Which Newcastle castle survives inside the modern city?

Which castle is meant by Newcastle's 'new castle', and why is the surviving site difficult to interpret?

Several phases are being conflated, from earlier defences to timber and stone castles and town walls, while the modern railway interrupts the surviving site.

Newcastle's name conceals several successive defensive sites and structures. Early remains, timber and stone castle phases, and the later town walls belong to different stages of the city's defences. Modern development complicates the surviving layout, with the railway passing through the castle area. Choosing the 'new castle' therefore requires identifying which historical phase and which surviving structure is meant. The dates are recalled informally and do not form a verified building chronology.