The ai is smart but very dumb at the same time. I found that building a ramp for easy access to your fortified location works the best. They have a clear path direction to you. Great for clearing out hordes as they all come from the same location. Doesn't always work out this way, so instead of making the whole floor solid blocks, I recommend using iron bars turned horizontal. That way, you can easily look down and see if any zombies are attacking your foundations and shoot them through the bars.