That's a 60° F HDD. Unfortunately, the HDD for the US tend to be based on 65° F. San Francisco has a mean of close enough to 3000 over 1948 - 2013 (it's probably lower now).
As these are temperature degrees, to convert from HDD 16 to HDD 65 (and ignoring that its 60 vs 65), that's 1310 * 9/5 that's 2358.
Ballpark, it needs less heating than San Francisco... though there's the "it's a castle and not well insulated" and "that's a large volume". However, it's not as big of an undertaking as one would imagine (to heat a castle) in a mediterranean climate.
I'd compare it to heating a church in San Francisco. Again, ballpark estimates put this in $1.00 - $1.50 per square foot.
Though, the "heating a room with a fireplace" wasn't the way it was done then.
https://solar.lowtechmagazine.com/2015/02/restoring-the-old-...
Also https://youtu.be/inzamhtroO0
> Kettle Moraine Heating & Air Conditioning installed solar panels designed to work with the brand-new Lennox systems and appliances used throughout the house. Now the owner—who bought the 10,000-square-foot historic home and updated the property with a 5,000-square-foot addition with help from Kettle Moraine—is saving a whopping $3,000 in electricity costs per year thanks to solar energy. And they’re pleased with the whole HVAC overhaul and new installation.
https://www.buildingmycastle.com/an-energy-efficient-castle/
Consider the equivalent question of "how do you heat Filoli Mansion?" When it was built, it was fireplaces and a diesel boiler.