This worked for us
1.) We made a hill - Enemies spawned on it.
2.) We built a house on the hill - Enemies spawned on it
3.) We built a moat around the hill with only a bridge - very few enemies appears in the base
4.) Surrounded the base with walls (after the moat) and gate - no enemies spawn in our base since then. We even now made extensions, new areas surrounded by moats and wall connected by bridge to our main base and no monsters spawn in those either.