Do NOT waste your time doing this. I spent like an hour or so getting this to look good around sanctuary without the clipping console command on the first day, and it sucked. Half the time enemies will spawn inside, and the other half the time they'll spawn outside and then not walk around to the entrances so both sides will just sit there shooting the wall like idiots. And wall or not if you don't go to the settlement to help it will fail (cause it doesn't actually spawn them of course ;p). Also don't build sentry turrets right next to each other, they have like no health and one missile will take multiple out in that case. Also put them ontop of a bridge piece or something so if it's ghouls or radscorpions they can't hit them.
Oh that sucks.
I though if you're bad was adequately defended then you wouldn't need to show up yourself.
Though damn ...the main I've shown up to was at The Castle and it was an army of high level supermutants charging over the walls along with their attack hounds..... no amount of turrets was going to help that ..... and they went straight for them with grenades!