A basic frame with drywall, and sometimes insulation to trap heat. For the outer wall, you can use brick, metal, wood, slatwall, or sidings.
Use a blowtorch to break into a metal base. Use explosives to break in, a sledge hammer, designated fire to make an entry point into a wooden wall by moistening the area around it, use a power tool to make a hole, or completely burn it down. Use explosives, a sledge hammer(Lots of hits to destroy wall), or use a a circular saw to make an entry point.
Defense using AI
You could tranquilize wild animals or lure zombies, and bring them into a fenced area that is around your base so they can defend it from raiders. Would make it hard to exit your base though.