A lot depends on how things work, mechanically, and in terms of AI.
For example, suppose I build my entire settlement on stilts so that, at the ground level, there is nothing but vertical columns and stairs leading up to the actual buildings. This might make it hard for bandits to get into my houses, but what if they can knock the pillars down? A few well-placed rockets and my entire city tumbles to the ground!
If at all possible, I'd like to have some natural / impenetrable wall against which to place my settlement's "back".... I hate the idea of being attacked from multiple fronts (look out behind you! Ah!).... there was one picture of a settlement that appeared to be on a waterfront, I think some of it actually extending out over the water. As I recall, Fallout games don't let anyone attack ~underwater~ or while swimming at all, so that would be a great way to keep that side of the settlement safe, so we could focus on the other 180 - 270 degrees, or so,
Again depending on how sophisticated the system is, I'd like to set up a series of walls. That is, have an outer perimeter that's simple and cheap, to just slow them down and keep out molerats and feral dogs and such (chain link, dilapidated picket fence).... then have an interior, proper, wall that could stand some beating and would mainly serve to funnel foes through one or two main entrances (one of which may actually dead-end in a kill zone).
Then inside THAT layer have a 'last ditch' defense of turrets and random pieces of cover (and tall towers to shoot over the first two walls)....