Just for the hell of it I think I am going to build a small defensive building to house me and 8 of my fellow settlers. Two to do the food and 6 loaded with top guns as my loyal troops!

Has anyone built any defensive retreats and have any pictures for inspiration?

I'm in the middle of creating a large platform to house my power armors, but nothing fancy. I may build something similar to your plan on the next level.

It's what I use Covenant for. Not like the place is great for a regular settlement, but it makes for a cool little walled compound personal estate.

Not sure if you think this is small or not, but this is Taffington Boathouse wkstrm edition: http://www.wkstrm.se/screen-dumps/fallout-4/settlements/

I have my fortress of solitude on the overpass above the settlement with the robots. A 3 or 4 story mid sized tower with no doors. There is a hangar bay on the 2nd level facing the overpass side and another hangar bay off the top going straight down. Crafting stations on the top floor but nothing else really. I meant to put generators in the basemant but its unfinished. A raider and a t-45 sit there.

I also built a boardwalk market on the island. Needs Christmas lights.

My retreat is Red Rocket. Don't need food or settlers, I live alone. I have a supply line from Sanctuary where the food grows, soldiers are defending, and I have stores. The good thing is Red Rocket is so close if I want to sell stuff I can head over there and sell it in my store and come back to peace and quiet at Red Rocket.

At some point I might bring a few "elite fully-armed" guards just for RP, but no stores or farmers.

The bonus of having no settlers at your base is that it never gets attacked.

Mine is Hangman's alley. I haven't opened 2 of the 3 entrances so there is only the one lone passage though the fruit trees, dogmeat and a couple machine gun turrets. My building has about 3 floors with the bottom being 4 tiles and the top being about 10 tiles.