Originally posted by Maelstrom:

A complete base defense has 3 components:

1) Work bench coverage to prevent spawns inside your base
2) An earthen barrier to prevent mobs from walking into your base (trench works fine, you can raise the ground with a hoe to do the same thing, or flatten the outside of your trench for a nicer look that leaves your base on elevated ground)
3) A wall to break line-of-sight from the outside so mobs don't aggro on you or your base structures.

you don't need workbench coverage in your base since almost all player built structure blocks (aka things you interact with or some furniture objects) block spawns in the same radius that workbenches do