This got me thinking about whether it's possible to have a network of self-sufficient settlements without fast travel. It sounds like a worthy challenge.
My first Fallout 4 character had a large network of settlements and occasionally one would be attacked – even when the Defence rating was higher than Food + Water. With fast travel I could easily "warp" to the settlement and wipe out the attackers.
With my second character I never used fast travel and avoided settling up even one populated settlement. Without fast travel I couldn't see how I'll be able to traverse across the map in time to "save" the settlement from attack, and could see my entire gameplay involving trekking from settlement to settlement dealing with raiding parties.
So – is it possible to create properly self-sufficient settlements which remain undamaged and happy – even if I never visit them after the original setup?