You can build the fanciest settlement you like.
Skyrim Realistic AI Detection - Olivier Doorenbos's mod for Skyrim helped me greatly in assembling a lot of information about the game settings and get on with this process faster. ~- Newest Fallout Vid! The settlers wouldnt just attack any enemy, I mean, a Deathclaw come calling and all the Settlers go after it? I'm on xb1 so my options have been more limited but i stumbled upon a hell of a gem of an combat AI mod. NPC pathfinding fixed? Hell, anything beyond ground level usually confuses the living heck out of them. For Fallout 4 on the Xbox One, a GameFAQs message board topic titled 'Settler Sandbox and AI Expansion questions'.
It's just one of fallout 4s lovely systems.