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.
This could be to easily exploited, and Unturned II bases shouldn’t need insulation and drywall. It doesn’t seem like Nelson is making the game that realistic. Making the walls would become more of a chore than anything, and is realistic for no reason. Otherwise, its ok.
Using things other than guns or explosives to raid would be too easy. There would need to be a way to balance it. Like low teir raiding to high teir raiding. Using high teir gear on a small base would be a waste, but easy. And you couldn’t use low teir stuff on high teir bases. This way everyone can raid fairly depending on there loot.
I like the idea of being able to reinforce the walls, especially when we remember that in Unturned II there will be hordes, so being able to fortify a weak wall with a metal coating can be more interesting than having to tear down the old one and put the new one.
As for using other types of tools to invade, I am a little against it, as it makes sense that players want to look a lot for rare materials to make explosives, while they can keep hitting AFK with a sledgehammer until, sometime, it breaks.
Already the idea of AI defending the base, I am in favor in part, seeing a wolf or even a tame bear seems interesting, but maybe it can be a little OP if a very determined player, or a clan of players, dominates 500 bears for defend your bases.