With my second character I never used fast travel and avoided settling up even one populated settlement. With fast travel I could easily 'warp' to the settlement and wipe out the attackers.
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. This got me thinking about whether it's possible to have a network of self-sufficient settlements without fast travel. The upcoming overhauled Survival Mode in Fallout 4 is reported to disable fast travel (this report was linked to by Bethesda).
