If you wanna be "cheap" about it, raise the ground to create impenetrable walls. It's expensive as hell in terms of the stone required to do so, but it'll give you the peace of mind to build freely and might as well serve as your actual walls once you're done. (I ended up building a massive plateau after beating the Elder because I got an even with trolls and they smashed my first base to pieces. Ended up serving me well!)

Or, as others have suggested, trenches. It's not pretty, but right now there are no ways to stop them other than to make sure you're away from your base at least once an hour, going by what lexikatt said. I certainly hope there'll be potions you can brew to temporarily stop them later on, because sometimes I just wanna sit in my longhouse with some friends and drink mead while chatting without the worry of invasions.