I'd say block the south, set up turrets north, and dig-in in central.
The thing is that, in practice, Clear Cover has a 75% chance of being at least three times as good as setting up turrets (the bonuses being basically identical, but it applies to all three areas). Blocking is possible, but I think either Raise Volunteers to have a reserve, having traps to further reduce their abilities, or Take The Fight To Them to get in an early shot, are all probably better.
Something like one of these:
[ ] Plan Fort Up w/Militia
-[ ] Southern Front
--[ ] Normal Security Detachment
-[ ] Central Front
--[ ] Heavy Security Detachment
-[ ] Northwestern Front
--[ ] Normal Security Detachment
-[ ] Reserve
--[ ] Militia (if recruited)
-[ ] Erect Defenses
-[ ] Clear Cover
-[ ] Raise Volunteers
[ ] Plan Fort Up w/Ambush
-[ ] Southern Front
--[ ] Normal Security Detachment
-[ ] Central Front
--[ ] Heavy Security Detachment
-[ ] Northwestern Front
--[ ] Normal Security Detachment
-[ ] Erect Defenses
-[ ] Clear Cover
-[ ] Taking the fight to them
--[ ] Heavy Security Detachment
[ ] Plan Fort Up w/Traps
-[ ] Southern Front
--[ ] Normal Security Detachment
-[ ] Central Front
--[ ] Heavy Security Detachment
-[ ] Northwestern Front
--[ ] Normal Security Detachment
-[ ] Erect Defenses
-[ ] Clear Cover
-[ ] Prepare traps: All Fronts
I think I prefer the Militia option as it has the most redundancies (both the others take risks, the ambush plan in an obvious fashion, the traps plan just risks wasting the Traps action given its not great success rate), but all those look valid to me.
EDIT: Thinking about it, the Southern Front is most vulnerable in all of the above so this version is also probably worth noting:
[ ] Plan Fort Up w/Southern Traps
-[ ] Southern Front
--[ ] Normal Security Detachment
-[ ] Central Front
--[ ] Heavy Security Detachment
-[ ] Northwestern Front
--[ ] Normal Security Detachment
-[ ] Erect Defenses
-[ ] Clear Cover
-[ ] Prepare traps: Southern Front
That's a safer option for people who don't want to get a militia, too, though I'm disinclined to it personally...I'd rather take a 50% chance at traps everywhere than a 90% chance at 1/3 the traps if going traps (and prefer militia to that anyway), but that's personal preference.
Doubling up on Erect Defenses would also be possible, but I think all of these are slightly better than that.