Generally, having a densely packed area inside the wall with workbenches covering all exposed land will keep spawns from occuring within. We've been observing ours for days now with no spawns occuring within the walls of the compound we built as a result. Even the regular invasions always spawn well outside the perimeter.

If there is even a tiny piece of land that is NOT under workbench coverage, that could be the culprit. From what I can tell the spawn points themselves are tiny, like pinpoint tiny.