Like others said, ignoring terrain irregularities or even using them as advantage is a good way to start (anew).

You can also clip various building pieces into same "slot", or snapping point, and inside of each other. This way you can make some stronger fortifications or even just walls (iron cage fits inside wooden wall), though they may not protect from all potential assailants like trolls perfectly (against trolls having multiple layers of short walls is fine, they can't really step over them).