Natural spawning walls allow mobs to spawn in front of them. Player placed ones are safe.If I might ask, what exactly is the difference between natural walls and constructed ones (besides the fact that you can harvest one but not the other, and if I recall correctly natural walls don't allow for proper housing)?
I've been doing a hardcore summoners run (where I don't fight any mobs at all until I get the Imp staff and then only use imps / other summon weapons to deal dmg) and player placed walls are essential to control mob spawn locations so I can avoid being attacked by them once I've carefully cleared an area. Still need to worry about worms, but that's a minor issue.
edit: and of course I need to worry about mobs hoiking themselves into 'safe' areas.. mostly slimes.