.... a settlement's perimeter defenses open fire every time the enemies next door respawn.
I like to put missile turrets on the perimeter to create a No Man's (or Monster's) Land that attackers have to cross (provided they don't teleport into the settlement center). There are several settlements that rarely get work done because the defenses keep triggering and all my settlers go charging out to see what set off the defenses. For example:
At Warwick Homestead, whenever the nearby Mirelurks respawn at the hatchery, BOOM! Every time I visit for any length of time, there's usually at least one such attack.
At Outpost Zimonja there's a Raider in a nearby trailer that deploys two gun turrets every time he respawns. The missile turret at the front gate is easily in range of both those turrets.
The Slog is just barely out of range of the Forger guard post. More on that in a bit.
Sunshine Tidings Co-op has a pair of Raiders in a shack not far from the backside of the settlement. A turret placed there will trigger every time either Raider steps out of the shack.
What really keeps setting off my perimeter defenses and the settlers' overreaction is that whenever Supply caravans enter or leave the settlement, they pass close enough that the nearby enemies attack the caravans -- which in turn brings them in range of the perimeter defenses and suddenly everyone is off to the races. Those kinds of attacks are VERY common occurrences at
Abernathy Farm
Country Crossing
Croup Manor
Finch Farm
Greentop Nursery
Oberland Station
The Slog
Somerville Place
Tenpines Bluff
and Warwick Station
This situation makes all those settlements almost constant combat zones.
Why would any settlers want to settle in a combat zone?