As suggested, build lots of workbenches throughout your base. Workbenches deny opponents the ability to spawn within their build radius. And be sure to check on them every several days. It just takes one annoying night spawn of a Skeleton archer who slowly plinks away bow shots at your workbenches from outside your walls/moat to quietly wreck several of them, leaving you suddenly vulnerable to spawns inside your base should a full-on raid trigger. You think you are totally safe, but suddenly there is unexpectedly stuff inside your base.