How To Make Biomes Safer in Valheim
One of the best ways to make a biome mostly safe in this game is to put Workbenches everywhere. I say mostly safe because you will sometimes occasionally get a mob spawn depending on the space between your benches. Workbenches that are too spaced out will cause enemies to spawn between them depending on how they load in.
The world loads in a giant circle around your character and sometimes it will not load in a Workbench that's just out of range which will leave a gap where enemies can spawn. Also, which objects the world loads first is not always consistent. It may load your Workbench after it does a call for enemies to spawn. Something else I have seen happen is when you have Workbench upgrades to extend the radius, sometimes it will populate the upgrades after it does a call for enemies to spawn. This happens all the time at my giant Meadows base (first screenshot below).
In my experience, you can be about 90-95% successful in disabling enemy spawns in a wide radius while still having the terrain look pretty. To increase your odds of success to 99-100%, you only need to place more Workbenches. Avoid using Workbench upgrades to increase the radius if you want the best odds of success.
Building A Workbench Half Underground (All Biomes)
One of my favorite ways to hide a Workbench is by digging a hole and placing it inside. Depending on how deep you dug, you can refill the hole and level the land without destroying the bench (as shown above). As long as a small portion of the Workbench is outside of the ground it won't break.
Another thing that is good about this strategy is most enemies won't be able to destroy the bench depending on how it's buried. The only enemies who can hit it are those with AoE attacks like Lox, Seeker Soldiers, Trolls etc.
Building Workbenches In The Trees (Swamp Biome)
Something you can do to make the Swamp Biome more safe is build Workbenches in the trees around the area. You will need to build a ramp or platform that lets you climb up into the trees, but it's easy to build the enclosures once you're up there.
Normally what I like to do is place a Workbench on a 2x4 platform and then build a small roof over it so that rain doesn't destroy it. The only enemy in the Swamp that will attack these is a Wraith, sometimes a Draugr/Skeleton Archer will shoot at them too but I find this to be rare and normally they only shoot at it when they are agro'd onto you and can't get to you.
Building Workbenches In The Plains
My method of hiding Workbenches in the Plains involves creating a pillar of solid ground with the Workbench inside of it. The reason I do it this way is because of Lox, their AoE ground slam will make quick work of any benches you hide in the ground. What I do is raise the ground four times and then level it out. I'll place a bench in the middle of the area and then raise it a few more times to cover the entire top of the bench (as shown above).
Building Workbenches in Mistlands
The Mistlands has a few more strategies to hide Workbenches, I really like using the Ancient Roots which are indestructible. Often times you can put a Workbench under a root without lowering the ground at all, then you can raise the ground to hide the bench better (as shown above).
Sometimes though you'll have to lower the ground a bit to place the bench, then you can raise the ground again to completely hide it. The only enemy that can destroy this bench is a Seeker Soldier (or a Gjall if it's not hidden enough). Hiding Workbenches between the large rocks is a good idea too, I normally do a large circle around my base in Mistlands and hide benches everywhere I can.
How To Make Ashlands Safe
I should start by saying that there is no real way to make the Ashlands 100% safe, but you can decrease the spawn of enemies in areas around you by placing Campfires. This is at best a short to medium term solution though because the campfires will get destroyed over time. Workbenches will not survive in this biome either due to the fire rain mechanic, anything wood left out without a Shield Generator will burn.
My long term solution for Ashlands involves digging holes and hatching a Chicken or an Askvin inside of the hole. You will want to cover the hole completely using stone or grausten so that no enemies will see the Chicken/Askvin but they'll know it's there. This will result in them walking around back and forth in the area but never attacking what's below them. A bonus of this is they'll be so distracted by the enemy below them... they don't even notice you much either! If you look at the picture below you can see the enemy is alerted but focused on what's below him.
I recommend you put these around your base in Ashlands - far enough away so that you can see them from your base but not so close that enemies will hear you or bother you while you're doing base activities. My advice is to put 4 around your base, one in each cardinal direction... Unless one side of your base is blocked off by lava... in that case do 3 around your base instead.
How To Make Deep North Safe