The workbench radius isn't really all that big. It's about 20m. The base items will only stop enemies from spawning inside their area, it won't stop any who spawn outside it and come to visit. During mob attacks, enemies can actually spawn outside that red circle and come to you. Because it's an event, they start off alerted and know your location so they'll always come to your base if you stay inside. If you're not talking about a mob raid and just general spawns... kinda the same thing. They won't spawn inside the area of your base item but unless you completely fill the surrounding area with workbenches or campfires or wards... you're always going to get spawns nearby.