Workbenches, wards and campfires all prevent spawns there are others too, but these are the main ones, but it is a radius, not a cylinder, so you need to have good coverage to prevent spawns, and heads up, enemies can spawn outside the radius, then move up, break the workbenches, therefore decreasing the radius.

in my experience, best defense is a wall, the inside needs plenty of fires and workbenches (ward are amazing too, as they reduce damage to the stuff you build, and will send out an audible blue pulse if something gets damaged, alerting you to unwanted guests)

Finally, tame some wolves, and have them patrol your perimeter ;D