I am playing Single-player mode.
After building my structure, I notice that no creatures are attacking.
I am just living in a house on a cliff all by lonesome self.
Is there a setting I need to check on/off or increase the level of hardness or something ?
Creatures don’t normally attack until you get within aggro radius of them. Additionally, if you have built your house on a cliff the pathing to that house might not be an easy one and the house might be far outside of aggro radius of any NPC/monster/animal. Now if you actually want a challenge, try putting a treasure coffer in your house. You WILL get people coming to “check it out”!
Just a heads up.
Be sure you’re not in creative mode, because that basically makes you invisible to NPCs.
As Eraden said, your base will not normally be attacked. If you are close enough to creature spawn points, they might attack you when you step out of your door/gate, but they will not randomly attack the base. (If you build really close to their spawn points, they will not spawn at all.)
You can build a treasure coffer, but that will only lead to a purge camp forming (if it is able to find a suitable location) if you ‘declare wealth’ - ie. you deliberately choose to attempt to summon a purge to your location. It will not happen at random.
If you really want your base to be randomly attacked, you could install Xevyr’s Purge Mod - this re-implements a version of the old purge where your base would be attacked periodically by a few waves of enemies - triggered by activity, built up on a meter, rather than by you choosing what level of wealth to declare to deliberately summon a purge.