Here is the relevant portion of the original sentence:
Its long, damp passages, its narrow cells and ruined chapel, were to be within her daily reach.
I would interpret it in this way:
Its long, damp passages (its narrow cells and ruined chapel) were to be within her daily reach.
→ Its long, damp passages were to be within her daily reach.
In other words, the second and third commas in the original version are used to represent parenthetical information that could be removed without affecting the syntax of the sentence.
If it's supposed to be describing three things that were to be within her daily reach, then the use of the third comma is somewhat misleading.
Instead, I would rephrase it in a few different ways:
- Its long, damp passages, its narrow cells and its ruined chapel were to be within her daily reach.
This removes the original third comma and adds its in front of the third item in order to preserve parallelism with the syntax of the first two items.
If you wished, you could add a serial comma after cells.
Its long, damp passages, its narrow cells and its ruined chapel: they were to be within her daily reach.
Its long, damp passages, its narrow cells and its ruined chapel—they were to be within her daily reach.
Both the colon and the dash add the kind of pause after the list of items that a comma normally would—but, unlike the comma, they don't indicate that anything is parenthetical.