An ornate Roman coin, medieval silver pennies, a copper figurine, a thimble, window glass and a key were unearthed.
Foundations were also discovered, showing the original structure of Auckland Castle in County Durham was "significantly larger" than thought.
Curatorial director Dr Christopher Ferguson said he was "really excited to have uncovered such a major finding".
It suggests the castle was not created as a manor house for the Prince Bishops of Durham, as previously thought, but that it had always been a large castle complex.
A five-month excavation has been carried out ahead of a new museum extension in the castle's Scotland Wing, formerly used to hold Scottish prisoners of war.
Durham University archaeological services manager Peter Carne said the work had "enabled an unprecedented amount" of research into the castle and its grounds.
The new museum, due to open in 2019, will look at the history of faith in Britain.
Archaeologists working at a 900-year-old castle have found "rare and unexpected" artefacts.