Each biom you visit has a chance to randomly spawn an attack event on your bases at random times. no matter where the base is located or what biom it is in. These events can happen multiple times throughout your play through. just use the raise ground tool to create walls around your base and the pick to make motes outside those walls and youll be fine. the forest is moving can be a pain early on if not prepared because the trolls can smash your wood and stone walls. which is why the mote is important keeps them away from your walls. or you can get really creative and build an island fortress like i did.