inside a castle; within the castle
Refers to the interior of a castle building or the area inside the castle keep.
Tours of the castle interior are conducted with a guide.
castle grounds; area within the castle walls
Refers to the entire enclosed area surrounded by castle walls, including courtyards and outbuildings.
Within the castle grounds there are several gardens and a museum.
The tourists formed a line and entered the castle grounds.
We toured the inside of the castle on our school trip.
Many retainers used to live within the castle grounds.