What is the technical term for the area within a fort?

Solution 1:

I think this area is called a bailey. A bailey can refer to the courtyard or the defensive wall that surrounds it.

Or it might be considered a ward, since it is inside the curtain walls.

And apparently, a ward and a bailey are the same thing.

Or it seems that it can be called a parade ground, as @F'x indicated and in agreement with this diagram:

UPDATE:

I was talking about this with a Dutch friend and he helped me find this page, which identifies area 38 as the vesting or fortress. Additionally, the entry identifies the flat terrain within the bastion, fort, or fortress as the terreplein. However, this is apparently the Dutch definition, since the English definition of a terreplein is substantially different.