Blanketing one's base with anti-spawn items will prevent them from spawning in your base. Creatures can spawn outside your base away from these items and work their way into one's base. I had necks spawning outside my base, and during storms when the waves were extremely high they were floating over my wall into my base. Had to double the height of that particular wall.
https://www.youtube.com/watch?v=_C9fKFg6bmw

Wards work in single player. I have active wards buried just outside my main gates, as they not only prevent spawns, wards also strengthen structures like walls, gates, cheval de frise. A gate within the proximity radius of an active ward in place will take quite a few more hits by a troll before busting down.