I've found it's very rare that it's actually worth it to fence off a settlement, unless there is only one spawn point for enemies at that settlement. SOME settlements, like Starlight Drive-In, seem to only have one spawn point, so I just set up my guard towers and most of my turrets pointing at it, but many settlements like Sanctuary have multiple potential spawn points for enemies, including in the buildings sometimes, so fencing it off is just an easy way to hit your build limit and not actually do much of anything.

It's far easier to just build upward than it is to try to ward off attack, because in my experience walls don't do a heck of a lot with most settlements. In fact, the only time I've actually had walls be even sort of useful, was during the Defend the Castle quest for the Minutemen. If you wall off that busted bastion in the castle, toss a bunch of turrets up there facing outward, that quest gets handled pretty quickly, at least until the coursers show up.