Here's a design idea for Sanctuary, inspired by the "hollow square" post-Crusades castle designs from Europe. Other than the 2x2 SS2 plots that the tutorials have you place, all other plots are interior plots built inside the walls, with non-SS2 items placed in the 3 houses that are inside the wall. Not shown are water purifiers in the river, that have some turrets protecting them. If you want to build bigger just add another floor on top. I used concrete for the bottom of the wall to avoid the bug where attackers can clip their guns through thin walls at ground level and fire at anyone inside. As your character level gets higher you can add more turrets as the attacks get stronger, but this design has been very successful at defeating attackers before they can do any serious damage.
The only mods used that are needed to build something like this are:
WSFW: required for SS2
Place Everywhere: recommended for SS2, used here to merge the wall into the existing houses that you can't scrap, cover the outer windows, etc.
To get this exact design you also need:
Craftable Turret Stands: more elegant than vanilla wall and floor pieces for placing turrets on exterior walls
IDEK's Logistics Station 2 and its SS2 patch: more caravan options than vanilla or SS2
I also recommend:
Settlement Attacks Beyond: attackers approach on feasible routes instead of teleporting into the middle of your settlement
SKK's Settlement Attack System: automatically repair damaged items (not sure if that includes SS2 plots, as I've never had one of them get damaged)
Personal Flyable Vertibird: not part of the settlement design, but since it's parked there I'll mention it is great for traveling like a boss