I've turned my sanctuary into a literal fortress and I'd love for it to get attacked. I've only had it attacked once in the 16 days of game play on this character.

theoretically storing water/food in a container (so it still builds up in the workshop) then when you have alot (like 10,000+) add it back to the workbench.

That should increase the chances your settlement will be selected for attack, as stored water/food is a factor in the formula used to determine if a settlement gets attacked.

Unfortunately i don't remember exactly what that formula was.

I think you have to allow conditions to be such that you are producing more food and water than you have defenses, but in practice it appears to be much more complicated than that would suggest.

Key "problem" with this game is randomness. For instance, one of my toons the other week noticed (for once!) an alert about Somerville Place being attacked. This is a settlement at which I'd made a few improvements but not much, and apart from a Provisioner, I hadn't added any setters -- so it was just the father and his two kids. When I get there, the siege is on and it's 5 or 6 Super Mutant Overlords, mostly armed with missile launchers. A stupidly over-powered force for attacking that particular settlement.

Next alert I get (and notice) is a different, but more fortified location, and the attackers there are basic Raiders armed with nothing especially powerful in terms of weapons.

I've also had a basically undeveloped Coastal Cottage attacked by an overwhelming force of Super Mutant Warlords, also with missile launchers.

Essentially, I don't see much logic behind any of it, and simply try to make the best of it. I currently have a 450 or so defense rating at a Spectacle Island base, and it's never been attacked. I want to see that, too. Even disabling the mirelurk acoustic harassment device did nothing.

Found it.

The formula for the attack change is: AttackChance = 0.02 + 0.001 * (FoodProduction + FoodInInventory + WaterProduction + WaterInInventory) - 0.01 * Safety - 0.005 * TotalPopulation.

It's evaluated daily. And yes, settlements without population will never be attacked.

More information https://www.reddit.com/r/falloutsettlements/comments/41key9/settlement_internals_explained/.

Edit: And according to the scripts, it does not matter whether you are member of the minuteman.