To solve the Father and Son safe code in Indiana Jones and the Great Circle, follow these detailed steps:

1. Complete the Fieldwork Side Quest “A Nun in Trouble”:
– Help Gina Lombardi rescue Sister Giuliana from captivity at the top of Borgia Tower.

2. Get Giuliana’s Key:
– After rescuing the nun, Gina will give you a key that opens a door to a balcony area in the tower.

3. Access the Balcony Room:
– Use the key to open the door and enter the balcony room. Inside, you’ll find a dog and a cat, but they are not relevant to the puzzle.

4. Find the Notes:
– In the main room of the balcony area, you’ll find two notes: an Italian poem with eight letters circled in red and a Polybius Square diagram showing nine numbers in a grid with six letters.

5. Analyze the Italian Poem:
– The poem contains eight letters that are circled. These letters are crucial for solving the puzzle.

6. Cross-Reference with Polybius Square Diagram:
– Use the circled letters from the poem and cross-reference them with the Polybius Square diagram to find their corresponding numbers.

7. Decode the Letters:
– The pairs of letters and their corresponding numbers are:
– I + R = 5
– I + A = 8
– T + A = 7
– S + O = 3
– Note that the handwriting can be tricky, especially for the last letter which looks like an “A” but is actually an “O”.

8. Combine the Numbers:
– Combine these numbers to get the final safe code: 5873.

9. Enter the Code:
– Enter the code 5873 into the safe to open it and claim your reward, which includes four more Mystery Notes and 160 Adventure Points.

By following these steps, you will successfully unlock the Father and Son safe in Indiana Jones and the Great Circle.